I just finally took a minute to get a general idea of the Cocoa classes. So in case it helps any other beginners, Cocoa framework classes can be roughly arranged as:
Application Kit: User interface, audio and graphics
Core Data: Building and using the CoreData data manager
timers times and timing
strings arrays dictionaries and formats
system inquiries and technical information
errors and exceptions handling
netwroking, ports, XML, URL, and files access
Key values, observing, notifications, queuing and protocols
This isn't a very good arrangement technically, but just something to give beginners a rough idea of the kinds of classes that are immediately available.
There are guides for just about everything in the Xcode documentation and online, for example::
Application Kit Framework Reference
Foundation Framework Reference
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