The T-1000 is a liquid-metal android, whose main goal is to kill John Connor in the past. Unlike the last Terminator, T-1000 can survive most any injury and changes back to it's original form almost instantly. He disguised himself as a cop and serves as the main antagonist in T2.
As the super-computer Skynet became self-aware, it started mass-producing the terminators, in order to wipe out the humans from the face of earth. A human resistance was led by John Connor, who organized the survivors of Judgement Day to fight the machines. Skynet then sent a terminator to the past to kill Sarah Connor before she gave birth to John.
As the first Terminator failed, Skynet took the next step and sent another terminator, the prototype T-1000, to kill John while a child. The future John Conner managed to capture a Terminator and sent him to the past as well to protect himself in the past.
Although the T-1000 is a heartless machine, he shows some traces of cynism and sarcasm and can act like a normal human being when in public.