what is heads up hold em ?

Answer 1

Playing only between 2 players
Heads Up Hold em is an variant based on texas hold'em

The Player may Raise his bet one time, and has three opportunities to do so. The earlier he raises, the more he can bet.

and  the differences between heads up hold em and ultimate texas hold em  = in the former the player may raise only 3 times his Ante  bet before the flop. but the game includes bad beat bonuses for losing with a straight or higher.