Ok this ain't a question but I thought it might come in handy for some newbies - a list of the majority of common file extensions (the letters after the "." of a file - to see them on your windows machine open a folder then go to view -> options. There should be an option that says something like "hide all known file extensions" un-check it and viola you can see them all)

.ai Adobe Illustrator file

.asp Active Server Pages. Pages created with ASP are dynamic and add interactivity to the web site.

.avi Movie files for use with the Windows Media Player.

.bak Backup files. This extension is commonly used in Windows as applications will make backup copies of documents with this extension.

.bat Batch files mainly used in DOS.

.bmp Bit-mapped files used for graphics such as Windows wallpaper.

.cal Files used with the Windows Calendar program.

.cbt Tutorial files usually used with Microsoft programs.

.cfg Files used to store information for MIDI components.

.com Command files mainly used in DOS.

.css Cascading Style Sheets

.dat Data files used by programs to store information.

.dll Dynamic Link Library files that help Windows program perform smoothly.

.doc Word processing document files most commonly associated with Microsoft Word.

.drv Device driver files that are used by hardware components to run smoothly.

.eps Files containing information designed for PostScript printers.

.exe Program files that actually run a program. Commonly associated with Windows applications.

.fla Macromedia Flash source file

.gif Graphics and picture files in a compressed format.

.html Website pages. HTML is an acronym for Hypertext Markup Language. Also website pages can end in .htm.

.ico Files representing icons such as Windows.

.inf Information files for programs during the installation process.

.ist Adobe ImageStyler file

.jpeg High-quality picture and image files. Also in .jpg extension.

.js JavaScript code files

.mdb Microsoft Access database file

.mid Sound files that contain musical notes. Also known as MIDI files.

.net Network files

.pdf Files that are created by Adobe Acrobat.

.reg Registrey key that contains licensing info to legalize the installation of an application.

.scr Windows screen savers

.sea Macintosh self extracting file.

.shtml Website pages that includes Server Side code.

.sit The Macintosh version of a compressed file.

.swp Temporary swap files for Windows.

.sys Windows system files.

.tmp Temporary files used by Windows.

.ttf True Type Fonts

.txt Text files created by a word processing program or Windows Notepad.

.wav Sound files that are commonly associated with Windows Media Player.

.win Backup files for the actual Windows operating system. Not to be confused with with .bak which are backup files for Windows application programs.

.xls Microsoft Excel document file

.z Unix compressed file.

.zip Files that have been compressed by a program such as WinZip or PKZip

Thanks to codecipient.com for the list

v_Ln