Detailed Explanation
BOB is a supernatural evil entity that exists in the Black Lodge, a dimension of darkness that exists alongside our world. BOB possesses human hosts and commits terrible acts through them, feeding on the "garmonbozia" (pain and suffering) he creates. He possessed Leland Palmer since Leland was a child, and through him, abused and eventually murdered Laura Palmer. BOB represents the show's theme that true evil can wear familiar faces—sometimes the monster is someone you've known your whole life.