As well as Cocoa and the Carbon High-Level Toolbox, there are many other APIs in Mac OS X. A lot of these will be useful from Cocoa, and are simple C APIs. The more modern of these use the standard CFTypeRefs which are of course TollFreeBridged to their respective Foundation classes. This is done much like CocoaGlossary.
System Frameworks come with the OS and are fine for you to use. Private Frameworks come with the usual disclaimer about expecting breakage due to the use of undocumented calls which may change in future releases or stop working and whatnot. Other Apple APIs are things which don't come installed as standard with Mac OS X but are add-on technologies.
To add content, type a page name here: [New]