File association

A file association associates a file with an application capable of opening that file. More commonly, a file association associates a class of files (usually determined by their filename extension, such as .txt) with a corresponding application (such as a text editor).

A single file extension may have several associations for performing various actions, also known as verbs. Some of the common verbs are:

A picture, for example, may be associated with these verbs so that open opens the picture in an image viewer, edit opens up an image editing program and print sends the picture to a printer.

Most operating systems support file associations in some form or the other. For example, opening a file from a file manager usually invokes the open verb in order to open the file with its associated application. The "open" verb invokes the associated application program, which reads the file or document and presents it to the user for viewing, and possibly for editing or other action.

Additional actions such as print are usually accessed via a right-click context menu. A mechanism for modifying associations is also usually present. An example for this is the Open With option in the Windows Shell.

The Microsoft Windows series of operating systems, beginning with Windows 3.x supports file extension-based associations. Associations are stored in registry as sets of verbs for each file extension. Older versions supported open associations stored in WIN.INI. File associations can be displayed and edited using the assoc command and with the help of the ftype command it is possible to create association between file type and application which will open files of this type.

...
Wikipedia