Majin Buu (transformations)
 Kid Buu
The original form of Buu that was created by Bibidi thousands of years ago. Kid Buu doesn't appear to have any sort of conscious and just loves to cause chaos and destruction. In that respect he is certainly the most dangerous form of Buu.
 South Buu
When Kid Buu invaded the Other World, he later absorbed the South Supreme Kai, transforming him into a tall adult Buu with extremely huge muscles. He gained a lot of power in comparison to his previous, genuine form.
 Fat Buu
Absorbing the Dai Kaioshin transformed South Buu into Fat Buu, which is actually the first form of Buu that we witness in Dragonball Z, but the third form that ever existed. Again, the "goodness" of the Kai made Buu even weaker, and made him more of jolly and care-free baby like creature.
 Good Buu; Mr. Buu
Good Buu (or Mr. Buu as some fans refer to him as) may look exactly the same as Fat Buu, but is significantly different because he has expelled all of the evil energy from within himself, making him a complete pure and "good" being.
 Evil Buu
When Fat Buu released the evil inside of him and became Good Buu, that expelled energy manifested into Evil Buu. In contrast to Good Buu, Evil Buu is tall, extremely skinny, and grey-skinned. Evil Buu wasn't seen for very long before he transformed.
 Super Buu
After a brief fight, Evil Buu absorbed the Good Buu, allowing the evil energy to take control and transform him into Super Buu. This version of Buu was taller, meaner, and had an obvious love for fighting as proved by his instant desire to fight Gotenks as soon as he was birthed.
However, even in this new evil state he had still shown some sort of compassion towards Hercule, by not killing him. This is also the first form of Buu that could sense power levels ever since the Dai Kaioshin was absorbed.
 Super Buu Form 2; Fusion Buu
When Super Buu realized he was unable to defeat Gohan, he absorbed Gotenks in order to make himself much more powerful. In terms of appearence, Buu gained more defined facial features (such as a nose for the first time), a much longer antenna, and gaining Gotenks' jacket.
He even learned some of Gotenks' techniques as well. This form of Buu only lasted for half an hour though, due to the fusion of Gotenks separating inside of him.
 Super Buu Form 2.5; Piccolo-Buu
Gotenks separated half an hour after being absorbed by Buu, leaving the monster to decrease in power and revert to this form, where Piccolo was the most dominant of his absorbed victims. The only difference for the way Buu looks here is that his fusion jacket has been exchanged for Piccolo's cape.