A file extension or filename extension is a suffix at the end of a filename. It is used to show the type of a computer file. This suggests what program can understand it. For example, MYWORDS.TXT is a plain text file and may be opened by a text editor. Filename extensions come after the name of the file, and many of them are three letters long.

Some examples of common file extensions are:

• HASH are hash program files
• TXT files are plain text files
• BMP are bitmap picture files
• JPG are picture files in the JPEG format
• MP3 are music files in the MP3 format
• AAC are Advanced Audio Coding music files
• FLAC are Free Lossless Audio Codec
• ALAC are Apple Lossess Audio Codec
• WMA are Windows Media Audio sound files
• MPEG are motion picture encoded video files
• MKV are Mastroska Video Codec
• WMV are Windows Media Video files
• HTM or *HTML are Hyper Text Markup Language files such as web pages
• PHP are web server scripts which create web pages
• ODT are Open Document text files
• DOC are text documents in Microsoft Word format
• XLS are Microsoft Excel spreadsheet documents
• PPT are Microsoft Power Point presentation files
• EXE are Microsoft Windows executable program files
• DLL are Dynamic Link Libraries, a type of executable file, in MS Windows
• ZIP are Compressed files (Lempel Ziff algorithm in an archive)
• Z are Unix / Linux compressed files
• bz2 are bzip2 block compressed files (very good compression)
• jar are Java archive files
• JSP are Java server pages files
• java are Java source code files
• class are Java compiled source code files
• oc are Java run-time library files
• tar are Unix / Linux tape archive files
• sh are Unix / Linux shell script files
• awk are Unix C like pattern processing language source code files
• sed are Unix stream editor command files
• lex are Unix / Linux lexical analyzer C code generator specification files
• c are C programming language source files
  • o are C programming language compiled (object) files
  • cs are C# programming language files
• rb are Ruby programming language files
• pl are Perl or Prolog programming language files

There are many other commonly used file extensions.