Domestique is French for servant. In the parlance of the peloton, this refers to a rider that goes back to the team car for drinks, clothes and food, then rides back up front to deliver these to his teammates.
But there is more to the term than just that. A domestique works for the benefit of his team and leader. He will support his team leader, pace him back to the peloton after mechanical faults, lead him tactically to the front and the end of the race. Cycling teams built around strong sprinters like HTC Columbia (Marc Cavendish) and Cervélo Test Team¹ (Thor Hushovd) form lead out trains, sprinting hard to so that their star sprinters can win a stage victory.
[¹] Although one of the world’s best sprinters Thor Hushovd is in the Cervélo Test Team, they also have a GC (General Classification) contender in Carlos Sastre, the winner of the 2008 Tour de France.