IOS
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iOS is the operating system on Apple devices such as the iPhone, iPad, and iPod. It was originally called "iPhone OS" because it was introduced for the iPhone (earlier iPods used a different system with much less "computer smarts"), but eventually it was rolled out in several other devices.
Development for iOS is done on Mac OS X systems using the Xcode development environment and the Objective-C programming language (with the Cocoa framework libraries). Development is, to some extent, easier in iOS than in the competing Android operating system because iOS runs only on Apple devices with a relatively limited range of screen sizes and capabilities, while Android runs on a wide variety of hardware and requires developers to consider the possibility of all sorts of differences. This distinction was even greater in the earlier days of iOS, when only one device type with one screen size existed; now it's not quite as true, as one must consider several screen sizes between the different iPads and iPhones/iPods.
Apple has a notoriously locked-down environment for its devices, where the only "legitimate" source of apps for iOS is through their App Store (and Apple also claims trademark protection on the name "App Store", leaving competitors scrambling for a properly generic term for a store where one may obtain apps). Developers can specially "provision" their own devices to allow them to test their apps on them, and can authorize a limited set of other users' devices to run their apps (e.g., for internally-used apps within a company, or for sharing them with a few friends), but otherwise, apps need to be distributed through Apple's store, which is arbitrary and capricious in accepting or rejecting apps based on its own rules.
However, there are unauthorized ways of "jailbreaking" iOS devices to allow them to use independent apps.
