A
ACE - ACE Archive compression file.
AIF - Audio Interchange File used with SGI and Macintosh applications.
ANI - Animated cursors used in Microsoft Windows.
API - Application Program Interface.
ARC - Compressed file format from an old program called ARC.
B
BAK - Backup Files.
BAS - BASIC programming language sourcecode.
BAT - MS-DOS batch file.
BFC - Briefcase document used in Windows.
BMP - Bitmap format.
BUD - Backup Disk for Quicken by Intuit.
BZ2 - Bzip2-compressed files.
C
C - C source file.
CAB - Microsoft cabinet file compressed for software (OS) distribution
CAT - Security Catalog file.
CBL - Cobol code.
CBT - Computer Based Training. Though for Mainframe programmers it might mean something else too!
CFML - ColdFusion Markup Language.
CGI - Common Gateway Interface. Web based programs and scripts.
CHM - Compiled HTML Help files used by Windows.
CMD - Dos Command File.
CPL - Control panel item - The settings in Control Panels represent CPL files in the Windows\System folder.
CPP - C++ programming language source code.
CTY - CityDesk site from Fog Creek Software.
CWK - ClarisWorks/AppleWorks file.
CWS - AppleWorks template file.
D
DAO - Registry Backup file for Windows registry.
DAT - Data file. Often viewable in a text editor.
DD - Compressed Archive by Macintosh DiskDoubler.
DEB - Debian packages.
DIR - Macromedia Director file.
DLL - Dynamic Linked Library. Microsoft application file.
DOC - Document format for Word Perfect and Microsoft Word.
DUN - Dial-up networking configuration file.
DWG - Autocad drawing.
E
EMF - Enhanced Windows Metafile.
EML - Microsoft Outlook e-mail file.
EPS - Encapsulated PostScript supported by most graphics programs.
EPS2 - Adobe PostScript Level II Encapsulated Postscript.
EXE - DOS based executable file which is also known as a program.
F
FFL - Microsoft Fast Find file.
FFO - Microsoft Fast Find file.
FLA - Macromedia Flash movie format.
G
GIF - Graphics Interchange Format that supports animation. Created by CompuServe and used primarily for web use.
GID - Windows global index. Contains the index information used by "Help" in Windows.
GRP - Microsoft Program Manager Group.
H
HEX - Macintosh binary hex(binhex) file.
HLP - Standard help file.
HT - HyperTerminal files.
HQX - Macintosh binary hex(binhex) file.
I
ICL - Icon Library File.
ICM - Image Color Matching profile.
ICO - Microsoft icon image.
INI - Initialization file used in Windows.
J
JAR - Java Archive. A compressed java file format.
JAVA - File containing Java source code.
JPEG - Compression scheme supported by most graphics programs and used predominantly for web use.
JPG - More common extension for JPEG described above.
K
KBM - Keyboard mapping script for Reflection 4.0
KDC - Kodak Photo-Enhancer image
KQP - Konica camera file
L
LAB - Microsoft Excel mailing labels.
LG - OOTP Developments saved games.
LGO - Windows 9x startup logo.
LHZ - Compressed archive by the LHA program from Haruyasu Yoshizaki.
LIT - eBooks in Microsoft Reader format.
LNK - Windows 9x shortcut file.
M
MAQ - Microsoft Access Query.
MAR - Microsoft Access Report.
MDB - Microsoft Access DataBase File.
MDL - Rose model file. Opens with Visual Modeler or Rational Rose.
MFM - MFWorks map file standard by Keigan Systems.
MHT - Microsoft single file web archive.
MPEG - Animation file format.
MPP - Microsoft Project File.
MSG - Microsoft Outlook message file.
N
NCF - Netware command File.
NDX - ArcView Fonts Index File by ESRI
NFF - Haines Neutral File Format
NFF - WorldToolKit Neutral File Format
O
O - Object file, used by linkers.
OBD - Microsoft Office Binder file. Ties Office docs together without having to load separate programs
OCX - ActiveX Control: A component of the Windows environment.
OFT - Microsoft Outlook template
OST - Microsoft Exchange/Outlook offline file.
P
PAK - WAD file that contains information about levels, settings, maps, etc for Quake and Doom.
PCL - Printer Control Language file. PCL is a Page Description Language developed by HP.
PCT - Macintosh drawing format.
PDF - Portable Document File by Adobe. Viewable in a web browser or with Adobe Acrobat.
PDF - Printer Description File. Provides printer support for certain applications.
PDR - Port driver for windows 95. It is actually a virtual device driver (vxd).
PHP - Web page that contains a PHP script.
PHTML - Web page that contains a PHP script.
PIF - Program Information File
PIF - Vector graphics GDF file(IBM Mainframe)
PIF - Macintosh Compressed archive
PL - Perl source code file.
PNG - Portable Network Graphic file.
PPS - Microsoft PowerPoint slide show.
PPT - Microsoft PowerPoint presentation(default extension).
PRN - A print file created as the result of "printing to file".
PUB - Microsoft Publisher document.
PWL - Windows Password list file.
Q
QBB - QuickBooks backup company file.
QBW - QuickBooks working company file
QIF - Quicken Import file.
R
RAM - RealAudio Metafile.
RAR - RAR compressed archive created by Eugene Roshall.
RAW - Raw File Format.
RDO - Raster Document Object. Proprietary file type used on Xerox Digipath Scan and Makeready workstations.
REG - Registry file that contains registry settings.
RM - RealAudio video file.
S
SCR - Screen Saver file.
SEA - Self-extracting archive for Macintosh Stuffit files.
SGML - Standard Generalized Markup Language.
SH - Unix shell script.
SMD - SEGA mega drive ROM file.
SVG - Adobe scalable vector graphics file.
SWF - Shockwave Flash file by Macromedia.
SWP - DOS swap file.
T
TAR - Unix Tape Archive.
TCW - TurboCAD Drawing File.
TGA - Targa bitmap.
TIFF - Tagged Image File Format. Universal graphics format supported by most graphics applications.
TMP - Windows temporary file.
U
UDF - Uniqueness Definition File. Used for Windows unattended installations.
UUE - UU-encoded file.
V
VBS - Microsoft Visual Basic script file.
VBX - Microsoft Visual Basic extension.
VM - Virtual Memory file.
W
WAV - Waveform sound file.
WMF - Windows Metafile (graphics format).
WRI - Write Document: This is equivalent to RTF, Rich Text Format, that enables saving text with formatting information.
X
XCF - The GIMP's native image format.
XIF - Wang imaging file. Wang Image Viewer comes with Windows 95/2000.
XIF - Xerox Image file (same as TIFF).
XIF - Image file eXtended by ScanSoft is similar to TIFF and is a Pagis application format.
XLS - Microsoft Excel Spreadsheet.
XSL - XML style sheet.
Y
YBK - Microsoft Encarta yearbook file
Z
ZIP - Compressed Zip archive.
Courtesy: techtutorials and many other online sites.