A set is a poker nickname for three of a kind. Another nickname for this hand is trips.
Some players assert that a set is in fact a special type of three-of-a-kind, where two of the three cards forming the three-of-a-kind are downcards and are therefore hidden, thus making it difficult for opponents to spot, while trips are any three-of-a-kind, including ones where two of the three are upcards. In Hold 'em, this effectively would mean that a set is three-of-a-kind where two of the three are in your hand, and the third is in the community cards on the board, while trips are hands where two of the three cards are in the community cards and only one is in your hand. When three cards of the same rank appear on the board this is referred to as board trips.
Although the value of three of a kind in and of itself is the same regardless of whether it's a set or trips, a lower ranked set can beat higher-ranked trips by virtue of the fact that the requisite board pair will complete a full house for the player holding the set. Unless there happens to be a second board pair (which would give the trips holder a better full house), a player holding any lower-ranked set will beat someone holding trips. In the opinion of some players, this makes sets strategically more valuable (especially after the flop and turn) compared to trips.
In common parlance, though, the line between using these two terms is becoming blurred, and even professional announcers are beginning to mix and match the separate meanings. It can be helpful to distinguish between the two situations (especially when discussing strategy), but it is becoming harder to simply assume that a reader will understand the difference merely by an author using one of these two terms.