Heads up play is a common poker term that means one on one. When playing an opponent heads up, psychology and luck become much more important skills than they would typically be at PokerStars.net. Psychology is important because of the fact that every move you make is being more closely scrutinized thanks to the fact that there is only one person to watch. Luck is also important because when you play heads up, you will definitely be playing many more hands to their end.
Psychology alone cannot win a heads up game, but it can come close. This is especially true of people that know each other very well. If you can pick up on the nuances of your opponent and translate what those nuances mean into which cards they are holding, you can know when to play a hand and when to immediately fold. You will also have a better indication of how much you can bet safely and not push them out of the hand when you have a monster pair of cards.
Luck is a huge factor in heads up play. Because you will see the flop on almost all hands, it is important that you catch cards often. In heads up play, you can play many more hands. Ace high or a low pair will often be good enough to take down the pot. Heads up play gets interesting though when both players are catching cards. This is usually how such a game would end; if both players have good hands, a couple all in calls is a definite possibility.