A framework is a collection of classes that are intended to be used together. That is, the classes are compiled together into a reusable library of binary code. Any related resources are put into a directory with the library. The directory is given the extension .framework. You can find the built-in frameworks for your machine in /System/Library/Frameworks. Cocoa is made up of three frameworks:
Foundation: Every object-oriented programming language needs the standard value, collection, and utility classes. Strings, dates, lists, threads, and timers are in the Foundation framework. All Cocoa apps, from command-line tools to fully-featured GUI apps, use Foundation. Foundation is also available on iOS.