Burn decks attempt to reduce an opponent's life total quickly using efficient spells that deal direct damage. Rather than depending entirely on creatures, Burn treats the opponent's life total as the primary target and tries to make every card contribute toward ending the game.
The original burn spell, Lightning Bolt, deals three damage for one red mana, while Lava Axe (the best burn spell ever, obviously) deals five damage for five mana. Shrapnel Blast also deals five damage, this time for two mana, with the additional cost of a sacrificed artifact.