• .doc - file created with MS Word and will usually be opened in MS-Word if you double-click on the filename in Windows Explorer
  
  
• .rtf - (Rich Text Format) will carry some, but not all, text & document formatting provided by MS-Word - should be opened in a word processor program
  
  
• .txt - text file - the simplest type of word processed document - contains no text or document formatting at all - can be opened by any word processing program including Notepad
  
  
• .xls - a spreadsheet file created with Excel and should be opened with this program or similar
  
  
• .ppt - a PowerPoint file
  
  
• .pdf - Portable Document Format - used to distribute pages created with desktop publishing software on the Internet - usually opened with Adobe Acrobat (free to download)
  
  
• .htm or html - either suffix is valid, the suffix(s) used for web (html) files, opened with an Internet browser such as Internet Explorer or Netscape Navigator
  
  
• .csv - Comma Separated Variable - contains data separated by a comma, hence the name - normally opened with a spreadsheet type program such as Excel, but can be opened by MS-Word as text without the formatting of a spreadsheet (e.g. in columns)

Compressed Files

• .zip - a file compressed into Zip format so using less memory and therefore quicker to upload, transmit over the Internet, and subsequently download - needs a program such as Winzip or similar, many of which are free

Image Files

• .psp - created using Paintshop Pro
  
  
• .bmp - Bitmap file - the simplest of graphic files - can be opened with just about any application including Windows Paint
  
  
• .jpg - JPEG - Joint Photographic Expert Group - a common file format used on the Internet - keeps file sizes to a minimum but as a consequence degrades the quality
  
  
• .gif - Graphic Interchange Format - another file format common to the Internet but not quite as degrading as jpg
  
  
• .tif -
  
  
• .wmf -

Audio Files

• .mp3 - the MPEG-3 audio file format, the most common format used to distribute digital audio - can be opened with Windows Media Player amongst others
  
  
• .wav - another digital audio file format - can be opened with Windows Media Player

Video Files

• .mpeg or .mpg - the standard video format used on DVDs and it's small size makes it suitable for Internet distribution of video files

Operating System specific Files

• .exe - executable file - contains a program that will run automatically if you double-click it - often it is this type that is hidden in an e-mail attachment as a virus
  
  
• .bat - a batch file - contains a string of commands designed to automate processes such as copying a hard disk - best left alone unless you really know what you are doing

There are more than this but these form the ones you are most likey to come across.