Magneto (born Max Eisenhardt) was a German Jew in Auschwitz Concentration Camp, his experiences there were the basis of his action in the decades to follow. He was a friend of Charles Xavier (around that time he discovered his mutant magnetism power) who would later become the founder of the X-Men, but parted ways due to their ideologies being too different. Some time later he formed the Brotherhood of Mutants as a mutant supremacy group which came into conflict with the X-Men a number of times, but has sometimes fought alongside them against a greater threat.
Magneto is, as his name of choice implies, truly a master of magnetism.
He can control all forms of magnetism, both artificial and natural. It is unclear, however, if he is able to draw magnetic force from outside himself, in which case he would be able to manipulate it over vast distances, or if he draws it from within himself. It is also unclear if his powers are psionic in nature, or purely physiological. It is possible that he merely focuses his power through his body. His power is, for all practical intents, boundless. He can even achieve certain tasks with his power simultaneously, such as completely assembling a complicated machine within seconds, and erecting a field of nearly impenetrable magnetic energy to protect himself from harm.
Even as his primary power is control over magnetism, Magneto has also exhibited control over the electro-magnetic spectrum. Gamma rays, ultra-violet rays, visible light, radio waves, and even x-rays are all at his command. But manipulating other forms of energy is more difficult for him, and thus he mostly sticks with magnetism. Magneto has also exhibited the ability of astral projection and telepathy, and claims to be able to control other minds. If this is true, his capacity along these lines appears to be blurred at best.
Another down side to Magneto’s power is that if his physical condition is dire, he cannot undergo the strain necessary to perform his abilities.