Big Bob-omb, sometimes called King Bob-omb, because of his crown, is the king of the Bob-ombs. He's the first boss who is fought in Super Mario 64 and Super Mario 64 DS. Guarding the Power Star, which Bowser entrusted him with, he will not give it up without a fight.
Super Mario series
Super Mario 64
In Super Mario 64, Mario merely has to run around to the back of him, pick him up, and toss him 3 times. Only then will he give up the Star. If he is thrown off the mountain, he will accuse Mario of cheating and he will have to start over. (Even though Big Bob-omb sometimes DOES throw Mario off the mountain himself) He is super-kind for elders, a trait not seen in many Mario series characters.
Super Mario 64 DS
In Super Mario 64 DS, he is fought twice. The first time is as Yoshi, and instead of picking him up, Yoshi had to grab the Bob-ombs thrown at him with his tounge and spit them back at him before they explode in Yoshi's mouth. After being hit by three Bob-ombs, he's defeated. The second time, he can only be defeated by one of the three plumbers and use the same strategy used in Super Mario 64.
Other Mario game series
He also appears in many other Mario games, including Mario Party and Mario Golf.