A file association is a relationship between a file type and a supporting application. For example, a Word document may be associated with Microsoft Word. This means when you double-click a Word document, Microsoft Word will open the file.
Both the Windows and Mac OS X operating systems use file associations to define what default program is used to open each file type. For example, in Windows, plain text (.TXT) files are commonly associated with Microsoft Notepad. In Mac OS X, they are associated with Apple TextEdit. Therefore, if you double-click a plain text file in Windows, it will open in Notepad, while in Mac OS X, it will open in TextEdit.
Since some file types have multiple file extensions, file associations relate each file extension to a specific program. For example, a file association for the HTML file type may require both .HTM and .HTML files to be associated with a specific Web browser. The file association for JPEG image files may require .JPG, .JPE, and .JPEG files to be associated with a specific image viewer.
In Windows, file associations are defined in the registry, while in Mac OS X, they are listed in the LaunchServices preferences. Fortunately, if you want to change a file association on your computer, you don't need to access either of these locations. Instead, both Windows and Mac OS X provide a simple user interface for modifying file associations. For instructions on changing file associations on Windows and Macintosh systems, visit the pages below.
NOTE: While you can manually change file associations using the methods above, some applications can also change file associations for you. For example, when you install or open a program, it may ask you if you want to make it the default program for all supported file types. If you select "Yes," the program will assign all supported file types to itself.