Bowser is the main villain, and also the "true" boss in the Mario universe. He can be shaken to be turned into Bowser Jr..
There can be several Bowsers in one level, with a majority of six (three in the main area, three in the sub area). Bowser can be turned into a bigger variation of himself when the player puts a Super Mushroom on him. Larger Bowsers can spit fireballs that can destroy blocks. He can easily be defeated by running into him with a Super Star, and he often appears next to where the level ends.
Bowser acts differently in each game style. In Super Mario Bros. style, Bowser simply jumps and spits fireballs. In Super Mario Bros. 3 style, Bowser can ground-pound, which can break blocks. In Super Mario World style, Bowser has the ability to breathe fire that rains down to the ground. In New Super Mario Bros. U style, Bowser acts like in Super Mario Bros. style, but he actually aims his fireballs at Mario. In a clown car in any style, Bowser throws Bob-ombs.
Bowser Jr. acts the same in all styles. He spits fireballs at Mario and will spin around in his shell if he is too far or if Mario jumps on his head. Jumping on Bowser Jr.'s head three times defeats him. If he has wings, pits don't harm him and he throws hammers instead of spitting fireballs. If he is in a clown car, he throws shells and only uses his fire/hammer attack after being attacked two times.