Bob-ombs are literally walking bombs. They appear as small black orbs with two white eyes and walk about on a pair of stubby legs. A clockwork key protrudes from the Bob-omb's back and rotates as it walks. On top of the Bob-omb is a fuse which is usually lit as soon as the creature starts moving. After a short while, the Bob-omb will flash red and then explode a second later.
The behaviour of Bob-ombs as differed greatly between the various Mario titles. Some games have shown them to be nothing more than mindless automatons that exist solely to pursue Mario and destroy him, but others have depicted Bob-ombs as sentient beings with their own society.