While the details are unknown as of yet (Damn Capcom always starting a spin-off before finishing the original), it is assumed that Mega Man X, or simply X, was an upgraded version of Mega Man, a service robot remodeled for war. Dr. Light, who created Mega Man, programed X to give Mega Man the gift of free will. Before he released X, he put him in a stasis capsle for 30 years to run tests to determine that X wouldn't breat any of the laws of robotics. 30 years later, X was found and reactivated by Dr. Cain. Robots were built based off of X's programing called Reploids. Everything seemed to be fine until a renegade Reploid harmed a Human. A Reploid who does that is called a Maverick. This robot was quickly destroyed, but soon other reploids went Maverick, and a Maverick Hunter force had to be assembled which was led by Dr. Cain's own best robot, Sigma. Unfortunately Sigma too went Maverick and X once again had to give up a life of peace to save the human race along with another Hunter, Zero.
Mega Man X, first game I ever got for the SNES and probably one of the best.