URL stands for Uniform Resource Locator


A URL is nothing more than the address of a given unique resource on the Web. 

In theory, each valid URL points to a unique resource. Such resources can be an HTML page, a CSS document, an image, etc.

In practice, there are some exceptions, the most common being a URL pointing to a resource that no longer exists or that has moved. As the resource represented by the URL and the URL itself is handled by the Web server, it is up to the owner of the webserver to carefully manage that resource and its associated URL.



Here are some examples of URLs

https://developer.mozilla.org
https://developer.mozilla.org/en-US/docs/Learn/
https://developer.mozilla.org/en-US/search?q=URL

Any of those URLs can be typed into your browser's address bar to tell it to load the associated page (resource).

A URL is composed of different parts, some mandatory and others optional. The most important parts are highlighted on the URL below (details are provided in the following sections):


Path vs URL 

The main difference between a path and a URL is that all “dividers/levelers” in the path are backslashes (f.e. C:\Windows\System 32\) but in URLs, there are slashes (f.e. http://www.google.com/).

Secondarily (in most cases) path refers to local resources inside your computer and URL refers to remote resources on some server. Additionally, the URL must be started with a type of communication protocol f.e. “HTTP://”. 

The local path usually starts with the letter of the partition (absolute path) or with a slash or system variable (relative path).



Valid vs Invalid URLs


Mistyped

If you enter a Web address in the location line of your browser by typing it in or pasting it from another source, you must enter it accurately to make the website page at that address load into your browser. Accidentally typing an extra character, or omitting one, within the address itself or within the authority -- the "www" portion -- renders the address invalid. Your Web browser passes on the address exactly as you provide it, and if your entry proves incorrect, your page won't load.


Incomplete

Just as transposed or missing characters in a Web address can render it invalid, so can the lack of critical parts of the address. In many cases, the website page you want to load comes from a file directory created within a directory within a directory. If you enter the correct domain name -- the google.com part -- and the proper page, but fail to include the precise file path to the location of the HTML page, blog post, or product specification page, the server that stores the files for the website in question replies by essentially saying "Huh? I can't find that information where you requested it."


Nonexistent

When you type in the address of a Web page that no longer exists or never existed, or a website that went offline when its owner shut it down, you might as well be trying to drive to an imaginary town on an imaginary street. If another website now handles requests for pages from the now-defunct site, the programmers of the new site may have set it up so it forwards your request to the new destination of your information. In that case, the invalid address you entered turns into a valid request for a page on a completely different site.

Invalid Characters

Web addresses must consist of letters, numbers, and a very limited set of punctuation marks, most of which divide up the addresses into their specialized components parts, such as the prefix -- the "HTTP://" part -- and the authority. If you enter characters that don't come from the accepted list, including accented vowels or consonants, spaces, and the rest of the punctuation marks, you create an address that can't exist the way you typed it. To use some of these characters, website developers can incorporate special codes into their page or folder names. For example, the code "%20," minus the punctuation, translates to a keyboard space. Because of the problems, these special characters can cause, however, most website developers avoid using them in website page addresses or the names of files that they make available for you to download.



Local URLs vs Server/ Accessible URLs

As a simple example, any URL like HTTP://localhost:8080 is a local URL. 

At the same time, any URL like https://testsigma.com is an accessible server URL hosted on the web