Bob-ombs (Bomuhei (ボム兵), "Bomb soldier" in Japan) are items that will explode on contact when hit or thrown. After a short time, they will start walking back and forth on the stage and then will stop, turn red and explode. In Brawl, picking them up activates the timer, causing it to explode in one's hand if not thrown in time. Bob-ombs are quite dangerous items, as they do severe knockback on contact with players. Even a player who grabs a Bob-omb must be careful, as it is very easy to KO oneself if one throws the item too close, or sometimes if one is struck by a projectile while holding the bomb.
In Sudden Death, Bob-ombs fall randomly from the sky if the match takes too long. These Bob-ombs are slightly larger than normal and (despite what most believe) can be grabbed through grabbing the item in the air. The falling Bob-ombs are absent in Super Sudden Death.
These little guys are just what they appear to be: walking bombs. Pick one up and hurl it for an incredibly potent attack. If no one picks them up, Bob-ombs light their own fuses and start wandering around armed and dangerous. In Super Mario 64, Bob-ombs chased Mario down once they noticed him. Pink Bob-ombs are nice and capable of speech.
A mobile bomb with a fuse on its head. It can be picked up and thrown as a powerful explosive, but if no one grabs it, it will tire of waiting and start wandering, then detonate whenever it likes. Its power and unpredictability make it an accident waiting to happen. In sudden-death matches that go on too long, Bob-ombs rain from the sky to move things along.