Cocoa is Apple's native object-oriented application programming interface (API) for the OS X operating system.

For iOS, there is a similar API called Cocoa Touch which includes gesture recognition, animation, and a different user interface library, and is for applications for the iOS operating system, used on Apple devices such as the iPhone, the iPod Touch, and the iPad.

Cocoa consists of the Foundation Kit, Application Kit, and Core Data frameworks, as included by Cocoa.h header file, as well as the libraries and frameworks included by those, such as the C standard library and the Objective-C runtime.

Site: http://developer.apple.com/technologies/mac/cocoa.html