Here is a short and incomplete listing of file extensions 

(The stuff after the period).

These are just some of file types that you may encounter on a typical system.

.ADF      Adapter Description File (file name extension)

.ADN      Add In Utility (file name extension) Lotus 1-2-3] [LDC]

.AFM      Adobe Font Metrics (file name extension) [Adobe Systems]

.ANI      Animated Cursor (file name extension) [Microsoft]

.ANN      Annotations (file name extension) [IBM]

.APP      Application (file name extension) [R:Base]

.ARC      Archive (file name extension)

.ARJ      Compressed File (file name extension) [Jung]

.ARTS     Cultural Groups (Domain Name) [Internet]

.ASC      ASCII text (file name extension)

.ASM      Assembler Source Language (file name extension)

.ASP      Active Server Page (file name extension)

.AU       Audio (file name extension)

.BAK      Backup (file name extension)

.BAS      Basic Language (file name extension)

.BAT      Batch Processing (file name extension)

.BGI      Borland Graphic Interface (file name extension)

.BIB      Bibliography (file name extension)

.BIN      Binary (file name extension)

.BK!      Backup (file name extension) [WordPerfect]

.B~K      Backup (file name extension)

.BLD      BASIC Bload Graphics (file name extension)

.BMP      Bitmap (file name extension) [Paintbrush Format]

.BSC      Boyan Script (file name extension) [Boyan Communications]

.C        C source code (file name extension) [C++]

.CAB      Cabinet (file name extension) [Microsoft]

.CAP      Capture (file name extension)

.CAT      Catalog (file name extension)

.CBL      COBOL source code (file name extension)

.CCS      Color Calibration System (file name extension)

.CDF      Comma Delimited Format (file name extension)

.CDT      Corel Draw Template (file name extension)

.CDX      Compound Index (file name extension) [Fox Pro]

.CFG      Configuration (file name extension)

.CGM      Computer Graphics Metafile (file name extension) +

.CHK      CHKDSK (file name extension)

.CLP      Clipboard (file name extension) [Windows]

.CMD      Command (file name extension)

.CNF      Configuration (file name extension)

.CNT      Contents (file name extension)

.COB      COBOL source code (file name extension)

.COD      Code List (file name extension)

.COM      Command (file name extension) +

.CPE      Cover Page (file name extension)

.CPI      Code Page Information (file name extension) [MS-DOS]

.CPL      Control Panel (file name extension)

.CRD      Cardfile (file name extension) +

.CUR      Cursor (file name extension)

.DAT      Data (file name extension)

.DBF      Database (file name extension)

.DCT      Dictionary (file name extension)

.DEF      Definitions + Defaults (file name extensions)

.DEM      Demonstration (file name extension)

.DES      Description (file name extension)

.DHP      Dr. Halo PIC (file name extension)

.DIC      Dictionary (file name extension)

.DIZ      Description In Zip (file name extension)

.DLL      Dynamic Link Library (file name extention)

.DOC      Document + Documentation (file name extensions)

.DRS      Driver Resource (file name extension) [WordPerfect]

.DRV      Device Driver (file name extension) (Also see .DVR)

.DRW      Draw + Drawing (file name extensions)

.DTA      Data (file name extension)

.DV       DESQview Script (file name extension)

.DVI      Device Independent (file name extension)

.DVR      Device Driver (file name extension) (Also see .DRV)

.DXB      Drawing Interchange Binary (file name extension)

.EDU      Educational Institutions (Domain Name) [Internet]

.EML      Electronic Mail (file name extension)

.ENC      Encoded (file name extension)

.EPS      Encapsulated PostScript (file name extension)

.EXE      Executable (file name extension)

.FAX      Fax (file name extension)

.FIRM     Business or Firm (Domain Name) [Internet]

.FON      Font + Phone + Phone Directory (all file name extensions)

.FOR      Fortran source code (file name extension)

.FRS      WordPerfect Graphics Driver (file name extension)

.GID      Topics (file name extension) [Microsoft]

.GIF      Graphics Interchange Format (file name extension)

.GLY      Glossary (file name extension) [Microsoft Word]

.GOV      Governmental (Domain Name) [Internet]

.GRF      Graph (file name extension)

.GRP      Group (file name extension)

.gz       Compressed file (GNU gzip compression/decompression program) [Unix]

.H        Header (file name extension) [C+]

.HLP      Help (file name extension)

.HQX      BinHex (file name extension) [Macintosh]

.HST      History + Host (file name extensions)

.HYP      Hyphenation (file name extension)

.ICO      Icon (file name extension)

.IDX      Index (file name extension)

.IND      Index (file name extension)

.INF      Information (file name extension)

.INFO     Information Services (Domain Name) [Internet]

.INI      Initialize (file name extension)

.JNK      Junk (file name extension)

.JPEG     Joint Photographic Experts Group

.JPG      Joint Photographic Experts Group

.LBR      Library (file name extension)

.LET      Letter (file name extension)

.LIB      Library (file name extension)

.LRS      Language Resource (file name extension) [WordPerfect]

.LST      List (file name extension)

.LYR      Lyrics (music file name extension)

.MAC      MacPaint (file name extension) + Macro (file name extension)

.MAI      Mail (file name extension)

.MAP      Linker Map (file name extension)

.MDF      Menu Definition File (file name extension)

.MID      Musical Instrument Digital Interface

.MIF      Management Information Files (file name extension)

.MIL      Military (Domain name) [Internet]

.MNU      Menu (fine name extension)

.MPEG     Motion Picture Experts Group

.MSG      Program Message (file name extension)

.MSP      Microsoft Paint (file name extension) [Microsoft]

.NCF      Netware Command File [Novell]

.NDX      Index (file name extension)

.NET      Networks (Domain Name) [Internet]

.NEW      New Information (file name extension)

.NOM      Individual with Personal Site (Domain Name) [Internet]

.OLD      Old version (file name extension)

.OPT      Options (file name extension)

.ORG      Non-Profit Organizations (Domain name) [Internet]

.ORI      Original (file name extension)

.OUT      Outlines (file name extension)

.OVL      Program Overlay (file name extension)

.PAK      Packed (compressed file name extension) [NoGate Consulting]

.PAL      Palette (file name extension)

.PAS      PASCAL source code (file name extension)

.PAT      Patch + Pattern (file name extensions)

.PCT      Picture (file name extension)

.PCX      Picture Image (file name extension)

.PDF      Printer Description (file name extension)[Borland,Lotus]

.PDX      Paradox files (file name extension) [Borland] +

.PFB      Printer Font Binary (file name extension)

.PFM      Printer Font Metrics (file name extension) [Windows]

.PGL      Graphics (file name extension) [Hewlett-Packard]

.PGP      ProGram Parameter (file name extension) [AutoCAD]

.PHO      Phone List (file name extension)

.PL       Pearl Language (file name extension)

.PLL      Prelinked Library (file name extension) [Clipper]

.PPD      PostScript Printer Description (file name extension)

.PPS          MS Power Point Slide Show file

.PRD      Printer Driver (file name extension) [Word]

.PRF      Preferences (file name extension) [Grammatik IV]

.PRG      Program (file name extension)

.PRJ      Project (file name extension) [Borland]

.PRO      Profile (file name extension)

.PRS      Printer (file name extension) [WordPerfect]

.PS       PostScript (file name extension)

.ps.Z     Compressed PostScript (file name extension) [Unix]

.PUB      Publication (file name extension) [Ventura]

.PX       Primary Index (file name extension) [Paradox]

.QDI      Quicken Dictionary (file name extension) [Intuit]

.QDT      Quicken Data (file name extension) [Intuit]

.QIF      Quicken Import File/Interchange Format (file name extension)

.QMT      Quicken Memorized List (file name extension) [Intuit]

.QNX      Quicken Indexes to Data (file name extension) [Intuit]

.RA       Real Audio (file name extension)

.RAM      Real Audio Metafile (file name extension)

.RAT      Rating (file name extension)

.RAW      Raw Data (file name extension)

.REC      Recorder + Recreational or Entertainment Activities (Domain Name) [Internet]

.REF      Reference (file name extension)

.RES      Resource (file name extension)

.SAV      Saved (file name extension)

.SBR      Source Browser (file name extension) [Borland]

.SC       Script (file name extension)

.SCR      Script (file name extension)

.SDF      Standard Data Format (file name extension)

.SEA      Self Extracting Archive (file name extension) [Macintosh]

.SET      Driver Set (file name extension) [Lotus 1-2-3] [LDC] +

.SHW      Show (file name extension)

.SIT      Stuff-It (compressed file name extension) [Macintosh]

.SMP      Sample (file name extension)

.SPL      Spell Checker (file name extension)

.SRC      Source (file name extension)

.SRP      Script (file name extension)

.STF      Structured File (file name extension) [Lotus Agenda]

.STORE    Businesses Offering Goods (Domain Name) [Internet]

.STY      Style (file name extension) [Ventura, Word, WordPerfect]

.SUP      Supplemental Dictionary (file name extension)[WordPerfect]

.SWP      Swap (file name extension)

.SYL      Syllabus (file name extension)

.SYM      Symbols (file name extension)

.SYN      Synonym (file name extension)

.SYS      System Configuration + System Device Driver (file name extension)

.tar      Tape Archive (archived file name extension) [Unix]

.tar.Z    Compressed Archived files (file name extension) [Unix]

.TBK      Toolbook (file name extension)

.TDF      Trace Definition File (file name extension) [OS/2] +

.TFM      Tagged Font Metric (file name extension)

.THD      Thread (file name extension)

.THS      Thesaurus (file name extension)

.TIF      Tagged Image File (filename extension)

.TLB      Table (file name extension)

.TLX      Telex (file name extension)

.TML      Template (file name extension)

.TMP      Temporary storage file

.TRM      Terminal (file name extension)

.TST      Test (file name extension)

.TTF      TrueType Font (file name extension)

.TUT      Tutorial (file name extension)

.TXF      Tax Exchange Format (file name extension)

.TXT      Text file

.UC2      Compressed File (file name extension) [UltraCompressor]

.UUE      UUEncoding [Unix]

.VAL      Validity Checks (file name extension) [Paradox]

.VBX      Visual Basic Extension (file name extension)

.VCC      Virtual Channel Connection

.VCD      Virtual Communications Driver

.VCL       Visual Component Library

.VFW      Video For Windows [Microsoft]

.VLM      Virtual Loadable Module

.VM       Virtual Machine + Virtual Memory

.VMM      Virtual Machine/Memory Manager

.VOB         DVD Video Object File

.VRS      WordPerfect Graphics Driver (file name extension)

.VXD      Virtual Device (file name extension) [Microsoft]

.WAV      Waveform Audio (file name extension)

.WEB      Entities Emphasizing the WWW (Domain Name) [Internet]

.WKB      Workbook (file name extension) [WordPerfect]

.WKE      Worksheet (file name extension) [Lotus 1-2-3] [LDC]

.WKQ      Spreadsheet (file name extension) [BORQU]

.WKS      Worksheet (file name extension) [Lotus 1-2-3] [LDC]

.WKZ      Compressed Spreadsheet (file name extension) [BORQU]

.WK1      Worksheet (file name extension) [Lotus 1-2-3] [LDC]

.WMF      Windows Metafile Format (file name extension) [Microsoft]

.WPD      Windows Printer Description (file name extension)

.WPG      WordPerfect Graphics (file name extension)

.WPK      Keyboard (file name extension) [WordPerfect]

.WPM      Macro (file name extension) [WordPerfect]

.WQ1      Spreadsheet (file name extension) [BORPQU]

.WQ!      Compressed Spreadsheet (file name extension) [BORPQU]

.WRI      Write (file name extension)

.XBM      X-Windows Bitmaps (file name extension) (black and white)

.XPM      X-Windows Pixelmaps (file name extension) (color)

.XLA      Add-In (file name extension) [Microsoft-Excel]

.XLK      Backup (file name extension) [Microsoft-Excel]

.XLT      Template (file name extension) [Microsoft-Excel]

.z        Packed file using Pack/Unpack program)..lower case z..[Unix]

.Z        Compressed file using Compress/Uncompress program)..upper case Z..[Unix]

.ZIP      Compressed File (file name extension) [PKWare]

.ZOO      Compressed File (file name extension) [Dhesi]