We Keep Coding

what happens if I ignore "coding nicely" and put css(using style tag) and js(using script tag) code in one html file

For about page, I'm just using a template from http://html5up.net/license for two reasons. One;there won't be many people checking my about page any way. two; I would rather spend my time on different thing and use free template.
Because I'm using the code I didn't write for this, I want this to be seperated. I don't want this to be interfere with my css and javascript folder. So, I added long css and javascript code in one html file. readability of code can't be worse than this, but it's just about page that I won't change frequently.
My question is if I add all of them in one html page, will it be a problem to my website's functionality?

It's actually recommended by some people to include all Javascript and CSS embedded in the page, even if it's repeated across many pages on the website.
The reason for that is because it reduces the number of HTTP requests to the server and makes you website's load-time much faster, especially when a user hits your website from a Google search result (one-time users). For web portals, it is not recommended because the user will continually browse through pages, and the Javascript/CSS is expected to be cached in a separate file.
Here's an example of how much slower a webpage can be with multiple external Javascript/CSS pointers:
If all those files were embedded into the page, the load time will decrease. Only after loading many pages on the same site will the repeated code will start to take effect.

I would suggest adding your code to an external stylesheet / script and linking that stylesheet & script in your document. eg.
<link rel="stylesheet" type="text/css" href="about-page.css">
<script src="about-page.js"></script>
This way you avoid messy code, and some common pitfalls that come with inline CSS / JS.
Or you can add to the existing CSS / JS files and add comments organizing your code, indicating what you added, etc...
There shouldn't be any huge problem, with exception of not being easily managed, but you really need to understand the 'cascade' in 'Cascading Style Sheets' to see what potential problems may arise.
It's important to remember that anything declared inline in your file holds more weight than things defined in an external stylesheet. So if a {text-decoration: none;} is defined in your external stylesheet, and you define a {text-decoration: underline;} in your html document. Your links will now be underlined, due to specificity.
To learn more about how the 'Cascading' aspect of CSS see Mozilla Developer Network - https://developer.mozilla.org/en-US/docs/Web/CSS/Cascade

Well sometimes using JavaScript or other bad practices to get an output that is "functional" my affect your SEO score which is used by search engines to recommend your page to users who might be searching for something you have.
Apart from this you will not be able to use technologies like gzip compression which greatly affect page load speed.
you might want to check:
Why page insights
gzip compression and caching
Website vs Web application
So simply a website is not only about functionality, it is about reach-ability, usability, and recommendation by search engines.

My question is if I add all of them in one html page, will it be a
problem to my website's functionality?
No.The effective result would be a Single Page Application

What does the script tag do

Yo everyone. I am Admin's relative. Just a beginner in javascript. I would like to ask what is the following line, its purpose, what it does, where you put it and so on. My question might be simple but it would be really much appreciated if anyone can give good explanation to this question.
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
Thank you very much.

As you can see, this loading jQuery library from the CDN.
what is the following line
This will include jQuery library on your page
its purpose, what it does
As you might have read jQuery Tag WiKi on SO,
jQuery (Core) is a cross-browser JavaScript library (created by John Resig) which provides abstractions for common client-side tasks such as DOM traversal, DOM manipulation, event handling, animation and Ajax.
You might want to visit jQuery homepage.
jQuery is a fast, small, and feature-rich JavaScript library. It makes things like HTML document traversal and manipulation, event handling, animation, and Ajax much simpler with an easy-to-use API that works across a multitude of browsers. If you're new to jQuery, we recommend that you check out the jQuery Learning Center.
where you put it
You can put this in head or at the end of body tag. You just need to add this before any jQuery plugin.
I'll highly recommend you to take this tutorial.

This line loads jQuery, a javascript library (see http://jquery.com for details on jQuery), in to your page. It is customary to put it in the <head> tag of your page, and as you're just beginning I would recommend that.
There are some cases, however, where you want to optimize your page load time and it may be better to put it at the end of the <bod> tag, right before </body>. But for now I'd put it in the <head>.
The src tells the script tag where to load the file from. In this case, it's from a CDN (content delivery network) which hosts the files, and is used for slightly faster download times (not something you'll need to worry about at this point).
And welcome to stackoverflow!

This line is usually placed in the header section of a webpage, but can always be placed anywhere depending on your need.
What the code does is it loads the javascript codes found at the link, onto the page.
This avoids having lengthy javascript codes within your webpage.

It is a link to an external script. It basically copies and pastes the code from that location into the file where you write it. You usually put it at the top of the file where you need it.

<script> tag is used for importing a script. (can be java script or vbscript.)
jQuery is a framework for JavaScript.
So the line you mentioned is to invoke the jQuery script code in your code.
Now, you can do it in two ways, either download it, and then give the src, or use it from cdn. The src in this line is from the CDN. For more details of how to use jQuery, the minimal you can read is: this.

It will load the content of the external script file. In this way you can better organise your code and reuse code libraries in different projects.
It is best practise to load the script files last in order for the browser to load the html first.

jQuery loading or appending scripts on specified page

Im a bit clules on the following situation, i am building a site, and right now all my js functions are in one js file, yes i know its a very bad idea.
What i am unfamiliar with, when i was checking other sites, i saw they include jQuery in the header and in the footer, different scripts are loaded.
I am unfamiliar, and please be nice i am a bit of a beginner.
Do people use some kind of plugin for this? or they include every script manualy in the specified file at the buttom?
I would like to break the scripts to parts, and not to include everything in one file.
What i mean by this, some functions only required in the profile page, some in the settings, and some in the login.
If someone could please give me some info about this would really help me.
Thank you

That's the sort of thing that one would use RequireJS for.
http://requirejs.org/docs/start.html
It will allow you to setup dependencies for your different JS files. These dependencies would then be loaded as needed.

Do people use some kind of plugin for this? or they include every script manualy in the specified file at the buttom?
Well, usually websites are made in PHP from individual components. Think of it as bricks. Some of these bricks contain JS code. When they are put together you end up with some pages having unique parts of javascript code in different places. It's not particularly good, but can dramatically simplify and speed up development.
I would like to break the scripts to parts, and not to include everything in one file.
Usually you want to include your JS with <script> tags in website header. If for some reason you'd like to dynamically include external JS then you may try to use this simple function: include();
i am building a site, and right now all my js functions are in one js file, yes i know its a very bad idea.
It's bad from development point of view, but when it comes to deploying the website it's a very good habit as it can decrease loading time of your website (and the loading time is one of most essential things). There are even applications compiling multiple .js files into a single minified file in order to get best performance.

Where should JavaScript be put?

I've been doing a little JavaScript (well, more like jQuery) for a while now and one thing I've always been confused about is where I should put my scripts, in the <head> tag or in the <body> tag.
If anyone could clarify this issue, that'd be great. An example of what should go where would be perfect.

Best practices from google and yahoo say that, for performance, javascript should always be put in an external file, and linked to your page with a <script> tag located at the bottom of the html, just before the closing of the <body> element.
This allows the browser to render the entire page right away, instead of stopping to evaluate javascript.

You mentioned three places:
In tags;
In the HTML; and
In an external file.
Let me address each of those.
Best practice is to have common Javascript in one or more external files and the less files the better since each JS file loaded will block loading of the page until that JS file is loaded.
The word "common" is extremely important. What that means is you don't want to put page-specific Javascript code in that external file for caching reasons. Let's say you have a site with 1000 pages. Each page has JS code specific to it. That could either be 1000 different files or one really big file that executes a lot of unnecessary code (eg looking for IDs that aren't on that particular page but are on one of the 999 others). Neither of these outcomes is good.
The first gives you little caching boost. The second can have horrific page load times.
So what you do is put all common functions in one JS file where that JS file only contains functions. In each HTML page you call the JS functions needed for that page.
Ideally your JS files are cached effectively too. Best practice is to use a far futures HTTP Expires header and a version number so the JS file is only loaded once by each browser no matter how many pages they visit. When you change the file you change the version number and it forces a reload. Using mtime (last modified time of the JS file) is a common scheme, giving URLs like:
<script type="text/javascript" src="/js/script.js?1233454455"></script>
where that mtime is automatically generated. Your Web server is configured to serve JS files with an appropriate Expires header.
So that mixes external files and in-page scripts in (imho) the best way possible.
The last place you mentioned was in the tag. Here it depends somewhat on what JS libraries and frameworks you use. I'm a huge fan of jQuery, which encourages unobtrusive Javascript. That means you (hopefully) don't put any Javascript in your markup at all. So instead of:
do stuff
you do:
do stuff
with Javascript:
$(function() {
$("#dostuff").click(doStuff);
});

Where do you put your javascript?

Do you localize your javascript to the page, or have a master "application.js" or similar?
If it's the latter, what is the best practice to make sure your .js isn't executing on the wrong pages?
EDIT: by javascript I mean custom javascript you write as a developer, not js libraries. I can't imagine anyone would copy/paste the jQuery source into their page but you never know.

Putting all your js in one file can help performance (only one request versus several). And if you're using a content distribution network like Akamai it improves your cache hit ratio. Also, always throw inline js at the very bottom of the page (just above the body tag) because that is executed synchronously and can delay your page from rendering.
And yes, if one of the js files you are using is also hosted at google, make sure to use that one.

Here's my "guidelines". Note that none of these are formal, they just seem like the right thing to do.
All shared JS code lives in the SITE/javascripts directory, but it's loaded in 'tiers'
For site-wide stuff (like jquery, or my site wide application.js), the site wide layout (this would be a master page in ASP.net) includes the file. The script tags go at the top of the page.
There's also 'region-wide' stuff (eg: js code which is only needed in the admin section of the site). These regions either have a common layout (which can then include the script tags) or will render a common partial, and that partial can include the script tags)
For less-shared stuff (say my library that's only needed in a few places) then I put a script tag in those HTML pages individually. The script tags go at the top of the page.
For stuff that's only relevant to the single page, I just write inline javascript. I try to keep it as close to it's "target" as possible. For example, if I have some onclick js for a button, the script tag will go below the button.
For inline JS that doesn't have a target (eg: onload events) it goes at the bottom of the page.
So, how does something get into a localised library, or a site-wide library?.
The first time you need it, write it inline
The next time you need it, pull the inline code up to a localised library
If you're referencing some code in a localized library from (approximately) 3 or more places, pull the code up to a region-wide library
If it's needed from more than one region, pull it up to a site-wide library.
A common complaint about a system such as this, is that you wind up with 10 or 20 small JS files, where 2 or 3 large JS files will perform better from a networking point of view.
However, both rails and ASP.NET have features which handle combining and caching multiple JS files into one or more 'super' js files for production situations.
I'd recommend using features like this rather than compromising the quality/readability of the actual source code.

Yahoo!'s Exceptional Performance Team has some great performance suggestions for JavaScript. Steve Souders used to be on that team (he's now at Google) and he's written some interesting tools that can help you decide where to put JavaScript.

I try to avoid putting javascript functions on the rendered page. In general, I have an application.js (or root.js) that has generic functionality like menu manipulation. If a given page has specific javascript functionality, I'll create a .js file to handle that code and mimic the dir structure on how to get to that file (also using the same name as the rendered file).
In other words, if the rendered page is in public/dir1/dir2/mypage.html, the js file would be in public/js/dir1/dir2/mypage.js. I've found this style works well for me, especially when doing templating on a site. I build the template engine to "autoload" my resources (css and js) by taking the request path and doing some checking for the css and js equivalents in the css and js directories on the root.

Personally, I try to include several Javascript files, sorted by module (like YUI does). But once in a while, when I'm writing essentially a one-liner, I'll put it on the page.

Best practice is probably to put it on Google's servers.
(Depends what you mean by "your" javascript though I suppose :)

This is something I've been wrestling with, too. I've ended up by using my back-end PHP script to intelligently build a list of required JS files based on the content requested by the user.
By organizing my JS files into a repository that contains multiple files organized by purpose (be they general use, focused for a single page, single section, etc) I can use the chain of events that builds the page on the back-end to selectively choose which JS files get included based on need (see example below).
This is after implementing my web app without giving this aspect of the code enough thought. Now, I should also add that the javascript I use enhances but does not form the foundation of my site. If you're using something like SproutCore or Ext I imagine the solution would be somewhat different.
Here's an example for a PHP-driven website:
If your site is divided into sections and one of those sections is calendar. The user navigates to "index.phhp?module=calendar&action=view". If the PHP code is class-based the routing algorithm instantiates the CalendarModule class which is based on 'Module' and has a virtual method 'getJavascript'. This will return those javascript classes that are required to perform the action 'view' on the 'calendar' module. It can also take into account any other special requirements and return js files for those as well. The rendering code can verify that there are no duplicates of js files when the javascript include list is built for the final page. So the getJavascript method returns an array like this
return array('prototype.js','mycalendar.js');
Note that this, or some form of this, is not a new idea. But it took me some time to think it important enough to go to the trouble.

If it's only a few hundred bytes or less, and doesn't need to be used anywhere else, I would probably inline it. The network overhead for another http request will likely outweigh any performance gains that you get by pulling it out of the page.
If it needs to be used in a few places, I would put the function(s) into a common external file, and call it from an inline script as needed.
If you are targeting an iphone, try to keep anything that you want cached under 25k.
No hard and fast rules really, every approach has pros and cons, would strongly recommend you check out the articles that can be found on yahoo's developer section, so you can make informed decisions on a case by case basis.