Bob-ombs are small bombs that explode violently. They are employed by Bowser to stop Mario in his quest against him (Super Mario series).
Bob-ombs first come into play in the Dry Bones Kart series in Dry Bones Kart 64. They were not used as an Item, but are sprawled out across the Track in multiplayer only. Also, in the Balloon Battlemode, if a player died in at least 3-player, they were transformed into Bob-omb Cars that can ram into surviving players.
After being absent from Dry Bones Kart: Super Circuit, they come back in Dry Bones Kart: Double Dash!! as Wario and Waluigi's special item. Petey Piranha and King Boo also can possess this item because they have the ability to get any special. Since then, a Bob-omb has been an Item in the succeeding games, open to every character.
In the game Dry Bones Kart 8, the Bob-omb will now walk around the track whilst ticking away. Oddly, the sound is the sound of it winding up instead of winding down. Previously, they only made ticking sounds while they sat on the track waiting to explode.