hi friend......
before explaining about file extension its good to know 1st about a File..
File-
A file is a collection of letters, numbers and special characters: it may be a program, a database, a dissertation, a reading list, a simple letter etc.
now what is a file extension-
a person belongs to a address.......same a file belongs to a reference that gives it a shape, a name, a well known address that actually called file extension...
in other way--
is the suffix at the end of a filename, which indicates what kind of file it is.
want to more go throw this link...
http://www.fileinfo.com/help/file_extension[
^]
Happy to help