iOS is the same operating system as used on the iPhone and iPod.
The iOS system is based on the same technologies used by Mac OS X, namely the Mach kernel and BSD interfaces. Thus, iOS apps run in a UNIX-based system and have full support for threads, sockets, and many of the other technologies typically available at that level. However, there are places where the behavior of iOS differs from that of Mac OS X.
See iOS App Programming Guide: The iOS Environment

Ver	Release date	Features	Highest version for
1	June 2007
2	July 2008	App Store
3	Jun. 2009	iPad support, Copy & paste, MMS, Spotlight search	iPhone (1st generation); iPod Touch (1st generation)
4	June 2010	Multitasking, FaceTime, Retina Display	iPhone 3G; iPod Touch (2nd generation)
5	Oct 2011	Siri, Notification Center, iCloud	iPod Touch (3rd generation); iPad (1st generation)
6	Sep. 2012	Replaced Google Maps,	iPhone 3GS; iPod Touch (4th generation)
7	Sep. 2013	Improved UI, AirDrop, Audio only Facetime over Wi-Fi, LTE, or 3G, Touch ID	iPhone 4, iPhone 4S, iPhone 5, iPhone 5C, iPhone 5S; iPod Touch (5th generation); iPad 2, iPad (3rd generation), iPad (4th generation), iPad Air; iPad Mini (1st generation), iPad Mini (2nd generation)
8	Sep. 2014	Support for iPhone 6 and Apple Watch, Updated camera app, Updated messages for audio and video and other enhancements. QuickType and alternate keyboards. iCloud drive support. HomeKit for home automation. Make and receive calls and messages on Mac and iPad.
9	Sep. 2015	Wireless CarPlay, Map enhancements, Music enhancements, New News replaces Newsstand.

* Note some features require a combination of hardware and software.
e.g. Siri requires an iPhone 4s or higher and iOS 5 or higher.
FaceTime requires an iPhone 4 with a front facing camera and iOS 4.

Sources:
iOS: A visual history | The Verge
iOS version history - Wikipedia

See also:
iPhone Comparison Chart : About.com
History of the iPhone - Wikipedia