There are some key differences to how you should play in a tournament setting as opposed to a ring game. As an example I'll provide the following scenario.
A relatively short-stacked player moves all-in, a second player calls and so do you. If this was a ring game, you and the remaining player in the pot with chips would do well to continue playing your respective games. However, if this was a tournament with a predetermined number of places "in the money," the incentive to knock the short-stack out of the game is higher than the possibility of increasing your stack from the other player.
If the short stack has pocket jacks and you have pocket eights and the flop is 2 3 7, you might be inclined to raise thinking your hand is solid. The raise may make the other player fold with his AQ. The turn is a three and the river is an ace. You lose and double up the short stack. Now if you could take back the raise after the flop, the AQ would have stayed in the hand and picked up the higher pair on the river. You still wouldn't win the hand, but the short-stack would be out of the tournament bringing you one step closer to placing in the money.
Of course, not everyone follows this advice, and from Mr. Short-stack's point-of-view it probably isn't fair, but it is good tournament strategy.
The exception to the rule? If you find yourself holding the nuts on the river, bet however you'd like.