IPhone OS 1

iOS 1 was the first iOS made. It is the main operating system for First generation iPod touches and First generation iPhones.

Similarities with iOS 5
iOS 1 is very similar with iOS 5 and iOS 4. Mostly, it lacked a lot of the newer features like Notification Center, Multitasking and Reminders. The user interface was slightly different too, namely the lack of a 3D-like dock. It also had a lot of wallpapers built in. The OS was definitely faster then today's, but at the time it was revolutionary.

Trivia

 * This was originally called iPhone OS 1.0
 * Before the name iPhone OS, Apple said the iPhone runs a version of OS X.