iOS (originally iPhone OS) is a mobile operating system created and developed by Apple Inc. and distributed exclusively for Apple hardware. It is the operating system that presently powers many of the company's mobile devices, including theiPhone, iPad, and iPod touch. It is the second most popular mobile operating system in the world by sales, after Android.
The iOS is the operating system that runs the iPhone, iPod touch, and iPad. It's the core software that comes loaded on all devices to allow them to run and support other apps. The iOS is to the iPhone what Windows is to PCs or Mac OS X is to Macs.
iOS shares with OS X some frameworks such as Core Foundation and Foundation Kit, however, its UI toolkit is Cocoa Touch rather than OS X'sCocoa, so that it provides the UIKit framework rather than the AppKit framework. Therefore, it is not compatible with OS X for applications. Also, while iOS shares the Darwin foundation with OS X,Unix-like shell access is not available for users and restricted for apps, preventing iOS from being fully Unix-compatible.
Major versions of iOS are released annually. The current release, iOS 9.3.3, was released on July 18, 2016. In iOS, there are four abstraction layers: the Core OS layer, the Core Services layer, the Media layer, and the Cocoa Touch layer. The current version of the operating system (iOS 9), dedicates around 1.3 GB of the device's flash memory for iOS itself. It runs on the iPhone 4S and later, iPad 2 and later, iPad Pro, all models of the iPad Mini, and the 5th-generation iPod Touch and later.