Generally, files will have a file name and an extension. For instance, in the example "classes.doc", classes is the file name and doc is the extension. Many programs use the extensions to find out certain information, such as what type of file is being examined, and what program was used to create the file. In the example above, doc represents a document made in Microsoft Word.

Extension	File Type	Explanation, or Associated Program
.doc	document	Used by Microsoft Word
.wp	document	Used by WordPerfect
.txt	document	TeXT file; this is the simplest document format because it does not use any special formatting codes
.pdf	document	Portable Document Format; used by Adobe Acrobat
.xls	spreadsheet	Microsoft Excel spreadsheet file
.exe	executable	A file which activates or runs a program, or the program itself
.bat	executable	BATch file (a script of instructions to be carried out)
.zip	compressed	A compressed file; use a program such as WinZIP to decompress it
.arj	compressed	Another type of compressed file
.cab	compressed	Microsoft's compressed file format (CABinet file)
.gif	graphic	Graphic Interchange Format; this is a commonly used graphics type
.jpg (jpeg)	graphic	This graphics file type works well for photos
.bmp	graphic	BiTmaP; this is a commonly used Windows file format; used for backgrounds and some clip-art
.pcx	picture	Picture format, mostly associated with digital camera files
.tif (tiff)	graphic/picture	A good file format for graphics and pictures, but it tends to have a large file size
.png	graphic	Portable Network Graphics format, this is similar to a .gif, but has a higher quality with a smaller file size
.ttf	font	True Type Font file
.wav	sound	WAVe sound file, commonly used by Windows applications
.au	sound	Another sound file format
.ra, .ram	audio	RealAudio sound files
.avi	movie	Movie file format, commonly used in Windows
.mpg (mpeg)	movie	Common movie/multi-media file type
.qtw	movie	QuickTime movie file format
.htm (.html)	hypertext	HyperText Mark-up language file format