In the poker game Texas Hold 'Em (and probably other games with community cards,) a set is a three of a kind hand composed of one community card and a pocket pair, that is, two pocket cards of the same rank. It differs from trips, two community cards and one hole card. It is an extremely strong hand in almost any situation.
Sets and trips are, of course, scored in exactly the same way: as three of a kind. However, sets are far more valuable!
So, let's say, hypothetically, you get dealt 7♠7♣, limp into the flop, and find, to your surprise and delight, that it is as follows:
3♥7♦Q♣
You are now in proud possession of a set. Lucky you! With your three sevens, you almost certainly have the best hand right now; only pocket queens beat you, and they are very rare. (In general, you don't have to worry about having a set beaten by a higher set.) Clearly, you are in a great position!
A set is beaten only by a straight, a flush, a full house, a better three of-a kind, or quads. Since sets can show up on any board, all of these are rare. There must be three or more of the same suit for a flush to show up, straights are rare unless the cards are clustered together, and the board must pair (that is, more than one of the same rank of card must show up on the board) for a full house or four of a kind to appear. Furthermore, a pair isn't necessarily a bad thing; while it makes a big full house or quads more likely, it also gives you a full house (or quads,) letting you beat straights and flushes. And those hands are unlikely anyway, even if the board pairs; the more likely case is that someone will get trips (or a flush, or a straight) and throw all of their money at you.
On this particular flop, all of these are unlikely. Unless the turn or river brings a pair, full houses and quads can't show up. The flop has cards of all different suits, so a flush is unlikely, as the turn and the river would both have to have the same suit to make one possible. Someone with 54 has an inside straight draw, but other than that every flush or likely straight has to come in through the back door, and they probably won't. Keep betting, and don't stop!
Even if this flop was more clustered or had two of the same suit on it, you still have the best hand, and there is a good enough chance that you will continue having the best hand at the end of the day to make betting worthwhile. A flush or open-ended straight draw doesn't come in that often, so your set will hold up a majority of the time. So keep betting anyway!
And I haven't even mentioned the best part yet. This is a completely ordinary flop! Anyone with a queen or decent pocket pair is probably feeling quite happy right now, not knowing that they are drawing to two outs; furthermore, if you bet furiously (and you should!) they won't immediately suspect a set, and will gladly call or even raise your bets.
Trips are a nice hand too, of course. Sure, they can be beaten by someone with the same three of a kind and a better kicker, but they are still three of a kind, and they still beat a bunch of different hands. The problem with trips is that with two same-rank cards sitting out on the table for everyone to see, you can be sure that everyone is thinking about trips, not just you.
A set is a monster hand. Trips are not.