File extensions are the three-character (or more) tags, separated by a period, that identify a file beyond its name: the ".exe" in Wordpad.exe, the ".doc" in First.doc, the ".bat" in Autoexec.bat, etc. With DOS and Windows 3.x, these file extensions are the link, or association, between an application and a data file. The file association is the computer's way of knowing which file belongs to which program, and the extension is the way it's shown.

If you have worked with DOS or Windows 3.x for long, you have probably figured this out. If you are in a directory and open a data file associated with an application, the operating system knows to open the application, then the document within it. Or, if a file itself is an application, the operating system knows to launch it. The operating system can tell that Wordpad.exe is an executable file, or application, rather than a data file. It knows that First.doc is a word processing document, so it opens the word processor, probably Microsoft Word, and then the document

With Windows 95 (Win95), however, the role of file extensions and associations changed considerably The association is separate from the file extension, and file names can include periods that do not denote file extensions.

With DOS, files are named with the eight-dot-three rule: The file name can be up to eight characters, followed by an optional period and file extension of up to three characters. The two items are separated by a period. The file extension is optional but pervasive.

File extensions are helpful for more than telling your computer how to deal with files. They also help you identify files - but only if you can interpret them. To see these extensions with Windows 95 Explorer, open the View menu, choose Options, and make sure Hide MS-DOS Extensions For Files Types That Are Registered is not selected.

.AVI Short for audiovisual interleaving. A motion-picture file commonly used with Windows Media player.

.BAT Short for batch. A text file that includes a list of instructions.

.BMP Short for bit map. A graphics file used for items such as the Desktop's wallpaper.

.COM Short for command. A file that gives instructions to a program.

.DAT Short for data. A file that stores information.

.DEV Short for device. A file used for a device driver. (Also .DRV)

.DLL Short for dynamic link library. A file that helps Windows programs operate more smoothly.

.DOC Short for document. A text-based compilation of data. Usually created with Microsoft Word.

.EPS Short for Encapsulated Postscript. The .EPS format allows for saving images with unconventional shapes, but the format takes up a lot more spce than other formats.

.EXE Short for executable.. A file that runs an application.

.GIF Short for Graphics Interchange Format. A compressed graphics file often used with the World Wide Web.

.HLP Short for help. A Text file that provides information.

.HTM Short for HTML, or Hypertext Markup Language. A text file used for the creation of World Wide Web pages..

.INI Short for initialization. A text-based startup file that tells the operating system which options are used in a given setup.

.JPG Short for Joint Photographic Experts Group. A compressed graphics file, usually a photograph, that is often used with World Wide Web.

.MID Short for MIDI, or Musical Interface Digital Interface. A sound file of music.

.PCD The Photo CD format is used by Kodak to store photographs on its Photo CD disks.

.PCT The Quickdraw Picture Format can save both bit-mapped and vector file informaiton. It is commonly supported by most Macintosh programs, as well as some some PC graphics and publishing programs.

.SYS Short for System. A file related to hardware.

.TIF Short for Tagged Image File Format. A common way to store graphics on PCs and Macintoshes.

.TMP Short for temporary. A file created, and usually erased, by Windows to protect information.

.TXT Short for text. A text file usually created by Windows Notepad.

.WAV Short for wave. A sound file that works with the Windows Media Player or Sound recorder.

.WMF Short for Windows Metafile. It stores both bit-mapped and vector formats, making it a metafile.

.ZIP Short for PKZIP. A file compressed with PKWARE Inc.'s PKZIP, which must be decompressed with PKUNZIP before the information isusable.