SNG in poker is an abbreviation for Sit and Gos, a form of poker tournaments that start when there are enough players registered. This differentiates Sit and Gos from classic scheduled tournaments, which begin at a specifically predetermined time.
SNG Poker Example:
A 6-max Sit and Go WSOP game will start as soon as there are six players registered. A six-handed game will generally pay only two players, distributing the prize pool 65% and 35% in favor of the first-placed player.