In the original movie, Halloween, Jamie Lee Curtis played Laurie Strode. The fictional character Laura Strode is a babysitter who is stalked on Halloween by a stranger who turns out to be an escaped mental patient named Michael Myers. The fictional character, Michael Myers, was played by Nick Castle.
Edit: Jamie's character, Laura, is Micheal's little sister in the movie. She's not just the baby sitter. She didn't know about her past until she was almost 20. Micheal killed his father, older sister, and her sisters boyfriend, but spared Laura because he loved her. The reason he goes back to attack her is because she rejected him.