Developed by Apple, iOS is distributed exclusively on Apple hardware such as iPhones, iPods and iPads. As of September 2015, it is the most commonly used operating system for mobile devices in major markets including Canada, USA and United Kingdom.
The first device to use the iOS system was the original iPhone, unveiled in 2007. Since then, the system has received numerous updates and its use has expanded across any Apple hardware that features a large capacitive touch screen for direct interaction.
The user interface is designed primarily around the concept of direct manipulation, where the user interacts 'directly' with the interface via touch gestures such as taps and swipes in order to effect sliders, switches and buttons.
Whlst this is now considered the primary method of interaction with any mobile user interface (UI), Apple have been widely been credited with being the first to bring these concepts into mainstream consciousness and effect a radical change in consumer technology that has widely influenced others.
Major revisions of iOS are announced annually. The keynote even at which this takes place has become a very important date in the calendar for technology enthusiasts and industry analysts.
iPhone OS 1
The first major release of the system had no official name, with Apple merely stating it was a version of Apple's desktop operating system, OS X. The statement was partly true, as while the code shares similarities which allows developers to more simply port apps to the mobile system it was in essence a siloed system.
Apple officially branded the system as iPhone OS on March 6, 2008, alongside the release of the iPhone software development kit. The OS was updated to the final version number 1.1.5.
iPhone OS 2
The second major release was made available on July 11, 2008 and was the first to support third-party applications via the App Store.
iPhone OS 1 users were able to upgrade for free on an iPhone or for the cost of $9.95 on iPod Touch. The OS was updated to version 2.2.1 as the final release.
iPhone OS 3
Released with the iPhone 3GS on June 17, 2009, this release added features such as copy-and-paste and MMS.
The original iPhone and iPod Touch could run updates to version 3.1.3.
Version 3.2 was relelased alongside the original iPad and was the first version of the system not to run on all previously compatible hardware.
Apple renamed the system to the simpler "iOS" when updating 3 to 3.2 for iPad, and 4 for iPhone / iPod on June 21, 2010.
Some features of iOS 4 were hardware specific so the older devices could not use them, including the iPhone 3G and iPod Touch 2nd Gen which could not use the multitasking capabilities or set a home screen wallpaper.
The iPhone 4, iPhone 3GS, iPod Touch (3rd & 4th generation) had all features enabled.
The originl iPhone and iPod Touch (1st generation) were unable to run iOS 4.0 and above.
Version 4 was updated to 4.2.1 which launched alongside the iPad 2 and was compatible with the iPad.
Released on October 12, 2011, the fifth version also dropped support for legacy phones in favour of features that required more advanced hardware to run effectively. This did not occur every year following, however has been a regular happening.
The iPhone 3G and iPod Touch 2nd Gen could not use iOS 5.
Apple included iCloud, their cloud-based storage service with iOS 5, as well as new home screen apps.
Version 5 was updated to 5.1.1, the final release supported o the original iPad and iPod Touch 3rd Gen.
Released on September 19, 2012 via iTunes (itself requiring an update to version 10.7), iOS 6 was selivered installed on new iPhone 5 devices.
Apple removed Google Maps and YouTube from the default installation, although they remained available as standard user downloads from iTunes.
OS 6.1.6 is the final release supported for the iPhone 3GS and iPod Touch 4th Gen.
Apple announced iOS 9 on June 8, 2015, with public release on September 16, 2015. Apple did not drop support for any currently up to date iOS devices. Therefore all devices supporting iOS 8 could run iOS 9.
Supported devices on this release include the iPhone 4S onwards, iPod Touch 5th Gen onwards, the iPad 2 onwards, and all iPad Mini devices. This made iOS 9 have the most supported devices out of any other iOS release, with 18 devices.
The iPad 2 is also the first iOS device to support six major releases of iOS, supporting iOS 4 to 9. This is the greatest amount of major iOS releases a single iOS device has supported, surpassing the iPhone 4S which supports five major releases (iOS 5 to 9), and iPod Touch 5th Gen supporting four major releases (iOS 6 to 9).
However, iOS 9 has limited support on the iPad 2, iPhone 4S, the iPad Mini 1st Gen, and the iPod Touch 5th Gen due to hardware limitations.