This phone will work with your active ATT Wireless plan. There is no contract required. Just place your existing sim card for your carrier into this phone and start using it! 4G LTE models may require LTE sim card, which is not included but will be provided by your carrier.
iPhone 4S Hardware Features
Among the most significant new features in the iPhone 4S are: Apple A5 processor - the same processor used in the iPad 2, which Apple says will make the iPhone 4S about twice as fast as the iPhone 4New dual-core graphics chip, which Apple touts as up to 7 times faster the iPhone 4
8 megapixel camera, with improved color fidelity and sharpness, better whites, a new light sensor, faster speed in taking both the first photo and subsequent ones