A lot of people seem to need classification on this lately.
A tiebreaker is used when two players in the same tournament group have the same score at the end of a round. The player with the highest tiebreaker score wins.
Tiebreakers are calculated via the formula:
(Score of opponents you win against) + (Score of opponents you draw against x 0.5)
Note that if you beat the same opponent twice, this means you will get twice his score added to your tiebreaker. (Once for each win.)
What's a tie-breaker (in the Chess.com tournament sense), and how does it work?