Tournaments are unlocked after clearing The First 19 Levels. To play a tournament one must pay an entry fee which varies depending on the difficulty level and tournament (lake is cheaper). However for all the tournaments there is no fee to play on easy level so for players starting out other than the campaign map tournaments are the primary way to earn gold for packs. Tournaments offer varying rewards upon completion, ranging from a bit of gold to packs to epic and legendary cards obtainable only from the tournament.
During a tournament the player must survive for three (four on hard) rounds with a set amount of health that does not replenish between rounds using a predetermined deck. Since it does not matter what cards the player has this is another reason why tournaments (lake and ruby in particular) are a great way for newer players to build a collection. Note also that winning a hard tournament requires a certain amount of luck (tempest in particular), but with proper play it is possible to win fairly constantly.
As of version 18.104.22.1689, there are five tournaments: