How does Mac OS X know what program to open when you double click on a file?

From what I understand, OS X decides by both file extension AND metadata (which is why if you get a .doc from a friend who edited it in TextEdit, it will have TextEdit's icon, not Word's.) Not sure how the precedence works exactly, though. What I do know is that it makes it damn hard to get consistent results for every file of a particular type.

There are programs that can change this after the fact, by the way -- you can do it in the Finder, of course (by doing Get Info on a document, selecting the application that should open it, and clicking Change All). If you'd like to have full control over the Launch Services database in one place, install http://www.rubicode.com/Software/RCDefaultApp/.

man 1 file

If you are asking for the purpose of changing the program that opens the file, Select the file and select File> Get info. Then, under Opens with... select the program you want to use and then click the button below that says something to the effect of "Apply to all." Incidentally, for internet protocols there is a separate Preference Pane you can download called More Internet.

RCDefaultApp is the only reliable way for a normal user to change these settings. The Opens with... thing in the Finder's Get Info rarely works when you try to apply the setting to all files of that type.