If you have a file in a generic format -- a text file, for instance, or a JPG image -- you have a choice of applications which can deal with it. You may want your text file to appear in a word processing application for thorough editing, for instance, or to pop up quickly in a text editor so that you can read the contents. You can control this on a one-off basis by right-clicking on the file name or icon and selecting 'Open with application...' from the local menu that appears. The menu will include some suggested applications, but you can override these and add your own.
But what if you want to associate a type of file with a particular program, so that double-clicking on a text file, say, will always open the default text editor (Gedit) rather than the Open Office word processor application? The secret is in the file properties. Right-click on the file name or icon and select 'Properties', then click on the 'Open With' tab. This allows you to add other applications to the list of potential applications, and to select one as the default by clicking the radio button to the left of that application. Any applications you add will appear from then on in the right-click menu, whether you select them as the default or not. If the application you want to use does not appear in the list you can add a custom command to bring it up. Easy!
Associating applications with document types 