When Kirby becomes Hammer Kirby, he appears with a blue bandanna on his head and wields a hammer. His hammer does massive damage. He can spin himself with the hammer in midair.
Super Smash Bros. series
Hammer is Kirby's side special in the Super Smash Bros. series. While it has gone through a number of changes between the games it generally is a powerful special with high knockback.
The Hammer is a pretty powerful attack on the ground though it's a bit slow. In Melee, if Kirby used it in the air, Kirby would spin it around himself but this was removed in Brawl. In Super Smash Bros. for Nintendo 3DS/Wii U, the move has an added charge ability to increase its damage called Hammer Flip.
Giant Hammer -a bigger slower but stronger version of the hammer, lacks the fire effect of the original charge