Heads up poker is a form of poker that is played between only two players. It might refer to either: A) a game that started out with just two players, or B) a game where only two players remain at a table or in a hand that originally had more than two players.
The rules of heads of poker are the same as in a game with more players, but the strategy employed is typically different from a multihanded game. Since only two players take part in the hand, the chance of having the best hand is much higher than in a multihanded game, which causes the game to become more aggressive than normal. Bluffs for example become easier to pull off in a heads up game since it is only neccesary to bluff a single opponent in order to win the pot, whereas in a multihanded game there is a greater risk of someone having a big hand that cannot be bluffed.
In poker tournaments heads up poker is played as individual events and there are also heads up championships. Heads up poker tournaments are typically played as knock-out tournaments. An example of a heads up tournament is the National Heads-Up Poker Championship.