A Swiss-system tournament is a competition format in which contestants are paired using rules designed to ensure that each competitor plays opponents with a similar running score (without playing the same opponent more than once). Match pairing for each round is done after the previous round has ended and depends on its results. The winner is the competitor with the highest aggregate points earned in all rounds. With an even number of participants, all competitors play in each round. It contrasts with an elimination tournament, where not all participants play in later rounds, as well as with round-robin tournaments, where each competitor plays every possible opponent.

(Thanks Wikipedia)