Various file types can effect both the behavior of your files, as well as how the server treats them. 

While there are numerous file extensions, which represent a host of various file types, we'll stick to the basic ones in this quick overview:
 

The .html file:

This is one is the most commonly used and the most one of you are already familiar with. Html stands for (hypertext Markup Language). Essentially, it tells the server, as well as the clients browser to process and display the .html coding in a way, which is meaningful to the end user through a browser.
 

The .htm file:

Many of you have probably noticed this newer extension appearing in place of the traditional .html one. In short, .htm is most often created, and or generated from the Microsoft FrontPage web editor.

The two are essentially the same and provide the same basic purpose. Unless you're using FrontPage, you will probably use the .html extension at the end of your web pages.
 

The .gif and .jpg file:

Most commonly used because of its good compression in web page images. Generally, .gif files are the fastest loading, as they remove a lot of information, which is not required to maintain image integrity, but to a point however. .jpg will allow more flexibility in compression and quality settings, however can also result in larger files.

The .CGI and the .pl file:

.cgi and .pl are most often used for perl scripts.

Perl scripts are small text based programs, which are executed on the server end, and will perform a host of interactive functions for a web site. 

In short, when a .pl or .cgi file is called, it tells the server to process it using the "Perl Interpreter." 

The Perl Interpreter understands the programming within the script, and will perform the set of sub routines, which will yield your desired effect. 

This desired effect could be anything from a simple web page counter, to more complex programs such as discussion forums, e-commerce platforms, to online auctions.