The Birth: Erik Magnus Lensherr a.k.a The White Pilgrim a.k.a Magneto's debut coincided with the debut of the X-Men in Uncanny X-Men #1.

The Legend: As a small boy, Magneto witnessed the of his family and his entire village by Nazis right before his eyes. He had his mutant powers even then, but his skill in using them were still in the infantile stage. An Auschwitz survivor, he is tormented by the knowledge that he could have prevented that and other war-time horrors if only he had realized his powerful mutant potential earlier in life. He has vowed that such crimes will never again be committed against his kind - that includes all mutants. According to his philosophy, the mutants called Homo Superior are the next step in human evolution and are destined to rule over the Homo Sapiens. Magneto is willing to use whatever force and take whatever steps he can, in the meantime, to protect the mutants and help them reach their rightful position. And with these ideals, he also wishes to enforce his will over all humanity.
Magneto met Charles Xavier in Isreal before Xavier set up his school for gifted youngsters and before Magnus turned evil. His main nemesis is Professor Charles Xavier and his X-Men. The X-Men exist to protect humanity and to help humans and mutants co-exist in peace and harmony; ideals which go against those of Magneto.
In his appetite for world domination, he also clashed with the Avengers, the Fantastic Four, the Defenders, Spider-Man and Thor.
He has also had partnership and battles with people like Namor, Dr.Doom and the Red Skull.
For a time, when Magneto turned good, Xavier entrusted Mags with the leadership of the New Mutants, just before Xavier left for space to recouperate from ailments. The New Mutants were a fresh batch of mutants who were recruited by Prof. when he thought that his original team had perished in battle. Magneto trained this band of young mutants and led them in the fight for justice, but then later reverted to his original self. The band then rejected Mags leadership and changed their name to The X Force.
Magneto was also responsible for formation of several oher mutant groups like the Brotherhood of Evil Mutants and the Acolytes .
In one of the biggest battles (not to mention landmark battles) against Charles Xavier, Magneto was defeated and the amalgamation of the evil with the two minds caused the creation of the creature known as Onslaught.

X-Men #1