Near as I understand it, Off-Broadway Equity contracts are for anything that isn't Broadway (duh!) in a NYC theater that seats more than 100. So, anything playing at a theater seating less than 100 is generally Off-Off-Broadway. Although it's not always that simple -- Fringe shows are considered O-O-B, even though some of the theaters they perform in are normally O-B houses.
Doubtless there are other posters on this site who can correct me or add more detail.