Fire Breath Move
Fire Breath is the B move of Bowser in both Super Smash Bros. Melee and Super Smash Bros. Brawl. Bowser continuously exhales a stream of fire on the ground in front of him, the stream getting smaller as he uses it until he's left exhaling smoke. In Brawl the Fire Breath can also be angled up and down, and never completely disappears.
This special attack is shared by Giga Bowser, both in his boss appearance in Melee and in his playable appearance in Brawl. There is no time limit to how long the attack can be held in his Melee incarnation. It also appears that a similar version of the move titled Flamethrower is used by the Pokémon Trainer's Charizard in Brawl. Both this move and Charizard's equivalent can be aimed up or down in Brawl.
Origin
Bowser's ability to breathe fire has been demonstrated in nearly every game he's appeared in, including his first game, Super Mario Bros., where he would shoot fireballs at Mario.