A default program is an application that opens a file when you double-click it. For example, if you double-click a .TXT file in Windows and it automatically opens in Notepad, then Notepad is the default program for files with a ".txt" extension. If the file opens in Microsoft Word, then Microsoft Word is the default program.
Default programs are necessary since many file types can be opened by more than one program. For example, your computer may have over a dozen applications that can open .JPG files. Therefore, the operating system needs to know which program to open by default when you double-click a JPEG image file.
Both Windows and Macintosh computers store a list of default programs for each file extension. These relationships between programs and file extensions are also called "file associations." Both the Windows and Macintosh operating systems allow you to change file associations if you don't like the default program that is associated with a certain file type. For example, if you prefer to play MP3 files in iTunes rather than Windows Media Player, you can change the ".mp3" file association to iTunes. This will set iTunes as the default program for all .MP3 files.
Windows 7 has a built-in utility for configuring file associations called "Default Programs." This tool allows you to assign specific programs to one or more file extensions using a simple graphical interface. It also displays what file extensions are associated with each installed application. For more information on using the Windows 7 Default Programs tool, view the FileInfo.com Default Programs Help Article.
While Mac OS X does not include a Default Programs tool, you can simply right-click a file and choose "Open With…" to select a different program to open it. If you want to change the default program for a specific file, select the file and choose . Then select a different program in the "Open with:" section of the window. If you want to change the default program for all files with the same extension, press the "Change All…" button.