iPhone OS 2 is the second major release of iOS. It was the first release of iOS to support third-party applications via the App Store. iPhone OS 2.2.1 was the final version of iPhone OS 2.
iPhone OS 2.0, the second major release of iOS, became available on July 11, 2008 with the release of the iPhone 3G. Devices running 1.x are upgradable to this version. This version of iOS introduces the App Store, making third-party applications available to the iPhone and iPod Touch. Prior to the public release of iPhone OS 2.0, Apple held a keynote event to announce the iPhone OS Software Development Kit ("SDK") to developers. More: en.wikipedia.org.