Apple describes the iPhone’s technology as layers. The base layer is the Core OS layer. On top of that layer is the Core Services. On top of the Core Services is the Media layer. The topmost layer is Cocoa Touch.

You can simplify the iPhone operating system (OS) even more; think of it as two layers—a C layer and a Cocoa layer. The C layer comprises the operating system’s layer. You use BSD UNIX–style C functions to manipulate this layer. This layer consists of things like low-level file I/O, network sockets, POSIX threads, and SQLite. The Media layer is also rather low-level and contains C application programming interfaces (APIs) like OpenGL ES, Quartz, and Core Audio. The Cocoa layer overlays the C layer, and it simplifies iPhone programming. For instance, rather than manipulating C strings, you use the Foundation framework string, NSString.