A term that refers to a player who plays a supporting role on his team, and does it well and willingly. A role player can either be a bench player who is a solid performer in the minutes he gets, or sometimes, a starter who plays a very specific role on the team.
Good role players are vital to virtually any team. They tend to be guys who come off the bench, don't play a lot, but are ready when called upon. They are usually students of the game, not the most talented players, but they make up for it with preparation and hustle. They give their team quality minutes in support of the team's stars, or in place of the team's stars so they can get some rest during a game.
The other type of role player is the kind who does play a lot, and may even be a starter. However, they are not star players, but ones who have a specialty they are very good at, such as rebounding. These players know their limitations very well, and don't do things on the court they aren't good at doing.