Add fragment and Query string for completeness

Fils System URL - A location on a PC

You have seen Absolute URLs like the following when loading files into your browser directly from your hard drive. 
   file:///Users/smith/Documents/file.html   (or C:\.... in Windows)
Such URLs are only valid on that particular computer.

Absolute URL - A Location on the Internet

   http://csci.lakeforest.edu/file.html
Technically, the IP address that csci.lakeforest.edu points to is a location on the Internet. The rest of the URL indicates a World Wide Web resource.

Absolute URL - Secure Delivery

   https://csci.lakeforest.edu/file.html
This is exactly the same, but all the packets are encrypted during transit. (Won't work unless the server is configured for secure transactions.)

Location Relative to Another Resource

   ../folder/file.html
The previous example shows how Web Browsers actually use these to transition between two Absolute URLs.

Location from the Web Site Root Directory

   /file.html
If a URL starts with a single /, that refers to the root directory of the particular Web site. Suppose you have surfed to a page several subfolders deep into a site like http://csci.lakeforest.edu/several/folders/deep/page.html. If a link in that page has href="/file.html" then file.html is in the root directory of the site.

It's quite similar for account based Web sites. Suppose you have surfed to http://csci.lakeforest.edu/~username/several/folders/deep/page.html. If a link in that page has href="/~username/file.html" then file.html is in your public_html folder since thats root directory of the user account site. In contrast, href="/file.html" refers to the root site of the actual server csci.lakeforest.edu rather than a particular account on it.

These "Site Root" URLs are actually very useful in practice, but that is beyond the scope of this course.

Absolute URL - Protocol Neutral

This is an Absolute URL, but does not specify the protocol -- http or https. 
   //csci.lakeforest.edu/file.html
This type of URL will adapt to either http or https depending on the context in which it is used.