Michael Myers brutally killed his sister when he was six, on Halloween. He was then put into a mental hospital and for 15 years he said and did nothing. Finally he escaped, and went to track down his only remaining family member, his younger sister Laurie Strode. She had been adopted by the Strode family and knew nothing about her link to Michael, since she had been an infant when the murder orrurred. With the help of Dr. Loomis, Laurie "kills" Michael and believes herself safe. She has a daughter, Jamie, who, as a child, has a psychic bond with her Uncle Michael. After Laurie is "killed" in a car accident, Jamie becomes pregnant (Part 6). She and her baby are kidnapped, and she is killed by Michael. In search of the baby, his last (so he believes) living relative, he terrorizes the Strode family, who is living in his house. Tommy Doyle, the child from the first one, finds Jamie's baby, and in the end beats Michael to a pulp, leaving him for dead.
In H20 Michael Myers is finaly decapitated by Laurie Strode, who it is revealed is not really dead, but living under an assumed identity in California. Of course, he's probably not really dead. Michael is most recognizable by his plain white mask with brown hair and his navy blue jumpsuit. He is sometimes called, "The Shape."
Part 6 [Halloween: The Curse of Michael Myers], explains how Michael is invincible and why he is how he is. There is an evil demon named "Thorn". The possession of the Thorn demon must occur in one family to save millions. The demon brings death and destruction wherever it goes. This all explains Michael Myers' tattoo on his arm near his hand.