One of the most powerful mutants of the Marvel Comics universe, Magneto has been the principle nemesis of Charles Xavier and the X-Men for years.
While most of comic-dom is obviously rooted in science-fiction, Magneto's powers make quite a bit of scientific sense. Of course, the idea that a human being could create and subsequently manipulate magnetic fields is a bit of a leap, but once we're past that, almost everything Magneto does in the X-Men comic books is explainable. One of the most common misconceptions held by readers of the X-Men comic books is that Magneto's powers are a work of BS, and that he should only be able to manipulate metals that are iron alloys. In other words, Magneto must work like your common fridge magnet. However, studying a little physics teaches us that this is not the case.
Magneto's main ability is that he can create and alter magnetic fields. Basic electromagnetic physics teaches us that an electric current and magnetic fields are closely related. The following two rules from physics are what give Magneto his ability to manipulate any conductor of electricity:
1) A magnetic field exerts force on electric current.
2) A changing magnetic field (in either direction or intensity) creates an electric current in a conductor.
So basically, by creating and changing Magnetic fields, Magneto should be able to create electric currents in metals. Once the electric current is there, Magneto can then use his magnetic fields to exert force upon the object in question.
The interesting thing is that this applies to any conductor. So, although this is never done in the comics, Magneto should be able to manipulate water as he wishes. In addition, since changing magnetic fields create electric currents in conductors, Magneto could probably electrocute a lot of careless people in pools if he felt like it.
One last point of interest. Organic matter (in fact many kinds of matter, in varying degrees) possesses a property known as diamagnetism. In the presence of a magnetic field, diamagnetic materials produce a magnetic field in the opposite direction. Scientists have used this property to float frogs in midair, as well as superconductors. So, in actuality, if Magneto really wanted to, he could lift us all up, despite the fact that we have no large concentrations of metal in our bodies.
While many comic book characters have powers that are just plain ludicrous in their magnitude and reach, Magneto's are actually rooted in strong science (even if the Marvel guys are unaware of it). If you really wanted to simulate Magneto's powers in real life, just start playing around with anything that makes a magnetic field, like a large solenoid or something like that. You could be tearing apart Sentinels or floating frogs in no time.