Aside from being a plant used to make chocolate (did you know that the cocoa tree is one of the few trees where the flowers and fruit grow directly out of the trunk and lower branches?), Cocoa is also the native API for MacOSX. Most Cocoa programs are written in ObjectiveC?, although there also exist bindings for Java.

The Cocoa application environment is designed specifically for MacOSX-only native applications. It is comprised of a set of object-oriented frameworks that support rapid development and high productivity. The Cocoa frameworks include a full featured set of classes designed to create robust and powerful Mac OS X applications. The object-oriented design simplifies application development and debugging. Cocoa provides developers starting new Mac OS X-only projects the fastest way to full-featured implementations. Applications from UNIX and other OS platforms can also be brought to Mac OS X quickly by using Cocoa to build state of the art Aqua user interfaces while retaining most existing core code.

Cocoa has its roots (so to speak) firmly in the NeXTStep? operating system, and is based on !YellowBox?, one of the NeXTStep? (perhaps Rhapsody?) APIs.

There is more to be said but this is a Linux wiki. Go read it. http://developer.apple.com/cocoa/. There's also an interesting history of Apple's operating systems at http://www.kernelthread.com/mac/oshistory/.