iOS is derived from Mac OS X and is a Unix-like OS. There are four abstraction layers within iOS:

Core OS Layer: Provides low-level features as well as frameworks for security and interaction with external hardware

Core Services Layer: Provides services required by upper layers

Provides services required by upper layers Media Layer: Provides the necessary technologies for graphics, audio, and video.

Provides the necessary technologies for graphics, audio, and video. Cocoa Touch Layer: Where frameworks are located, which are often used when creating an application

iOS comes with a lot of default apps, including an email client, a Safari Web browser, a portable media player (iPod) and the phone app.

Developers can use the iOS software development kit (SDK) to create applications for Apple mobile devices. The SDK includes tools and interfaces for developing, installing, running and testing apps. Native apps can be written using the iOS system frameworks and the Objective-C programming language. Included in the iOS SDK is Code Tools, which include an integrated development environment (IDE) for managing application projects, a graphical tool for creating the user interface and a debugging tool for analyzing runtime performance. It also includes an iOS simulator, which allows developers to test apps on a Mac, and an iOS developer library, which provides all the necessary documentation and reference material.