iOS was introduced in 29th June 2007 when the first iPhone was developed. Since then iOS has been under gone many upgrades and currently the latest one is the iOS 6. Apple has still not allowed any other manufacturer to lay hands on its operating system. Unlike Android, Apple has more concentrated on the performance rather than appearance. This is the reason that the basic appearance of iOS is almost the same as it was in 2007. Overall it is very user-friendly and is one of the best operating systems in the world.
So far iOS has been used in iPhone, iPhone 2G, iPhone 3G, iPhone 4, 4S, iPhone 5, 5S, 5C, not to mention their tablet pcs branded as iPad 3, iPad 2 and iPad.