Spoiler Ghosts/spirits of former victims, although "it" can assume the form of people the haunted person knows. It's never really explained. "It" gets passed from person to person through sex; the only way to get rid of it is to have sex with a different person and have them try to pass it on to someone else. But if the person you pass it on to dies, it reverts back to you. Sort of an evil spirit STD. I wasn't as impressed with this as everyone else seems to be. It's well done for a low-budget movie and has a wonderful creepy atmosphere (think "Carnival of Souls") but the script could have used some work.