A file extension is three or four characters at the end of a filename, such as .DOCX, .PDF or .TXT. File extensions tell you the file type, and which programs can open it. These extensions are used in Microsoft Windows, Mac OS X and Linux Operating Systems.

The most known file extensions

text files

.docx	Microsoft Open Word XML Document
.doc	Microsoft Word Document
.txt	Plain Text File
.rtf	Revit Family Template File
.odt	OpenOffice/StarOffice File

audio files

.mp3	MP3 Audio File
.wav	Wave Audio File
.aac	MPEG-2 Advanced Audio Coding File
.wma	Windows Media Audio File
.m4a	MPEG-4 Audio File

video files

.avi	Audio Video Interleave File
.mp4	MPEG-4 Video File
.mov	Video Clip
.flv	Video File
.mpg	MPEG 1 System Stream

data files

.pdf	Portable Document Format File
.xls	Excel Spreadsheet File
.csv	Comma Separated Values File
.ini	Initialization/Configuration File
.html	Hypertext Markup Language File

compression files

.zip	ZIP File
.rar	WinRAR Compressed Archive
.7z	Compressed File
.tar	Consolidated Unix File Archive
.gz	GNU Zipped Archive File

executable files

.exe	Windows Executable File
.msi	Windows Installer File
.bin	Binary Disc Image
.app	Punch Post
.dmg	Disk Copy Disk Image File