Foundation is an important part of Apple's API. It is enviroment independent and has such things as NSString, NSMutableArray, and NSTimer. You'll notice that before all it's objects it has NS. This is becaused it is all inherited from NextStep. NextStep was used to develop Rhapsody, which could run on many platforms. Now granted that Foundation only works in Apple's proprietary ObjectiveC, if Apple allowed it, it could run on other platforms. Up to date documentation may be found at