Net Terms

Extension

In computer operating systems, an extension is an optional addition to 
the name of a file.  It appears as a suffix in the form of ".xxx" 
where "xxx represents a number of alphanumeric characters.

The file name extension serves several purposes.  For instance, it 
allows a file's format to be described as a part of its name, so that 
users can quickly understand and recognize a particular type of file 
without having to open it or use it.  The extension also helps an 
application program recognize whether a file is a type that that 
program can work with.

For example, take the file mdevitt.doc.  The extension "doc" lets users 
know that the file is a Microsoft Word document without having to open 
the file and see what it is.  It also lets the Microsoft Word program 
know that it is a file the program can open and operate with.