In trick-taking games, to ruff means to play a trump card to a trick (other than when trumps were led). According to the rules of most games, you must have no cards left in the suit led in order to ruff. Since the opposing players are constrained to follow suit if they can, even a low trump can win a trick.
In many games, a player who cannot follow suit is not required to ruff. He may instead discard (play any card in any other suit).
Normally, ruffing will win a trick. But it is also possible that a subsequent player will overruff (play a higher trump). This is not always a bad thing?see uppercut (bridge).
In partnership games such as contract bridge, you give a ruff when you lead a card in a suit in which your partner is void, so that he can take the trick with a trump card. With luck, your partner will be able to get the lead back into your hand (by leading a certain suit) so you can give him another ruff. When your partner gets back to you by leading a void suit of yours, it sets up a crossruff.
Normal suit play in bridge (as opposed to the play at no trumps) revolves around the trump suit. Usually the declarer and dummy together will have the majority of trumps, as they chose the suit in which to play. Declarer will attempt to draw the opponents' trumps, leaving them with none. Declarer's remaining trumps ensure that the opponents cannot establish long cards, as they will just be trumped.
Although drawing the opponent's trumps is nearly always to be recommended, there are occasions when other strategies yield more tricks. One is crossruffing ? drawing the opponents trumps in this case reduces the number of trumps (and hence tricks) for the crossruff. Another case is when after drawing one or two rounds of trumps the opponents are left with one master trump. In this case drawing it will use two of declarer's trumps for one of the opponents'. Unless entry problems are feared, it is usually better to let the opponents take their trump when they will.
It is important to realize that trumping in the hand with more trumps does not add tricks, as these are long cards which will win anyway. In order to gain tricks by trumping, the ruff has to be taken in the short hand, or enough ruffs must be made in the hand which was originally longer in trumps to make it shorter than the other hand.