Any of you unfamiliar with file extensions will likely still recognize common extensions like ".doc", ".xls", ".pdf", or even ".mp3" . Windows uses these extensions to determine a file's type, and thereby which application should be used to open it. Mac OS 9 didn't use file extensions at all; it kept type and application info in a small extra file 'attached' to a given file called a "resource fork". Mac OS X now uses a sort of hybrid system, which I confess I haven't ever fully investigated, but for the most part file extensions are the rule (though they are often hidden from the user's view).
I recently learned of the file extension matching my initials: ".kmz". It's being used for a saved workspace in Google Earth. These files are actually zipped containers of several smaller files. Google acquired the technology behind Google Earth from a company called Keyhole, so I'm guessing the file extension comes from something like "Keyhole Map Zipped".
All in all, I could think of many much less interesting file types to have my initials on.