What is Cocoa?*

1. Cocoa is an application environment for both the Mac OS X operating system and iOS, the operating system used on Multi-Touch devices such as iPhone, iPad, and iPod touch. It consists of a suite of object-oriented software libraries, a runtime system, and an integrated development environment.
2. The various layers that constitute Cocoa are shown in the diagram: It is quite self-explanatory in terms of a over view. Do not want to get into details.
   

• How Cocoa fits into iOS?

• Here the point is that the Cocoa Touch along with UIKit are "Application Layer" elements for iOS that replace the Appkit of the Mac OS stack. 
• Foundation library continues the be the same.They form the core frameworks that we will have to work with for iOS development.

1. * All of this is from Apple documentation but put together so that it acts like a starter kit