A filename extension is the mostly 3 letter word that you see after the period in a filename. Since Windows 95, extensions can be larger than 3 characters. Extensions are usefull for determening the type of a file. There must be thousands of extensions. Some are well-known and used very often, others are used just once by a single program.
Programmers are free to make up their own extensions.
On this site I try to make an inventory of extensions, sorted in alphabetical order. You might think this site is usefull if you encountered a file somewhere and you don't know what to do with it. There's a good chance it can be found in the list, and if you're lucky there's also a shareware suggested for viewing or converting it. If not, maybe the description will help you to look further. Some MIME types can be found in the extensions base.