Cocoa Developer Documentation

Cocoa has two core object frameworks: Application Kit and Foundation.
Application Kit provides a hierarchy of classes that support a graphical user interface, event handling, and general application behavior. Foundation is a collection of "base" classes that specify fundamental object behavior and mechanisms for the deallocation, storage, notification, persistence, and distribution of objects.

Java Framework Reference Obj-C Framework Reference
Application Kit (HTML)
Foundation (HTML)
Application Kit (HTML)
Foundation (HTML)
Understanding Cocoa
Click here for additional documentation for Cocoa.