What is a file extension?

Answer 1

A file extension is a three character designation that tells software about the format of the data within the file. For example, a ".rtf" file is in rich text format which requires a word processor for it to be interpreted and a ".pdf" file is in portable document format (which requires Adobe Acrobat to interpret).
A file extension is used in some operating systems to identify the type of the contents of the file. It is usually separated from the main part of the file name by a ".", but some operating system have used different characters.

Most file extensions today are 3 characters long (e.g. txt, exe, jpg, mpg), but different operating systems have used file extensions as short as 2 characters to as long as 80 characters.

Some operating systems do not make use of file extensions. For example Unix makes no use of them at all. VMS and the original MacOS used a file type field in the directory entry instead of file extensions.

Answer 2

File extensions are suffixes that show what type of file it is. For example, a basic text file has the extension .txt. Sometimes they're shortened versions of the word, like .txt, but sometimes they stand for something, like .png (Portable Network Graphic). If you want to see file extensions and are on a Windows computer, open up Windows Explorer and select Organize > Folder and Search Options > View > and uncheck "Hide extensions for known file types".

For more info, see here. http://en.wikipedia.org/wiki/Filename_extension