Majin Buu was originally crafted from raw energy by the intergalactic magician Bibidi. Designed as a killing machine, Majin Buu was a mass destroyer of worlds, even going so far as to attack its own creator. In an effort to calm Buu down, Bibidi set his sights on the Supreme Kai, the elite overseers of the universe. Buu killed the North and South Supreme Kai, later on using his ability to absorb enemies while gaining their traits and abilities to attack the West Supreme Kai, changing in form significantly. Just as Buu was about to kill the East Supreme Kai (later on introduced as Shin or simply the Supreme Kai) the Grand Supreme Kai (or Dai-Kaio) stepped in, sacrificing his life to be absorbed by Buu. To Bibidi's shock, Buu took on a stout, docile form most in part to the Grand Supreme Kai's kind nature. This transformation also made Buu easier to control.
Time passed and Bibidi's creation was eventually sealed away by the remaining Supreme Kai. Buu remained sealed away to the time when Goku returns to Earth for the World Martial Arts Championship. The Supreme Kai informs Goku, Vegeta, and the other Z warriors of Babidi, a descendant of Bibidi, who intends to take control of Buu once again by gathering energy from strong fighters. Due to an attack on Gohan during the championship along with Goku and Majin Vegeta's bout, Buu is released, still in the same form as after he absorbed the Grand Supreme Kai.
Buu proves to be extremely powerful, using his ability to transform enemies into food on the Demon King Dabura and eating him. After easily dispatching Gohan, Buu is attacked by Vegeta. In a selfless act, Vegeta uses his Final Flash attack to cause Buu to dissipate into the air, while also sacrificing his own life. As the dust clears, Buu is very much alive, simply splitting into hundred of pieces that easily regenerate. Buu reunites with Babidi (who is cut in two by Piccolo), heals him, and the two run rampant throughout Earth.
Goku, upon awakening from his unconscious state from Vegeta's power, steps in to buy time for his son Goten and Vegeta's son Trunks as they practice the Fusion Dance. During his fight with Majin Buu, Goku transforms into a Super Saiyan 3, while also expending his time left on Earth. Buu, sick of listening to Babidi's bossy demeanor, kills Babidi and continues to attack Earth.
In a ridiculous effort to quell Buu's destructive nature, alleged World Champion Hercule appears. Offering gifts (along with some explosives) Hercule plans to eliminate Buu. However, he later on befriends Buu. When Hercule is shot by criminals, Buu, in a fit of anger, expells a puff of smoke that later materializes into the evil essence that Buu was just beginning to destroy. The newly formed Evil Buu successfully kills and eats the original Majin Buu, causing a raging transformation into a stronger, more maniacal Majin Buu, dubbed Super Buu. Super Buu then uses his Human Extinction attack to kill almost everyone on Earth.
When Goten and Trunks' training is complete, Piccolo leads Super Buu to the Hyperbolic Time Chamber where Goten and Trunks are training. Super Buu proves to be a powerful enemy, even regenerating from the smoke after the fused Gotenks' Super Ghost Kamikaze Attack. Piccolo, falsely convinced by Gotenks that Buu is too strong, destroys the door to the chamber, trapping all three of them in the chamber's empty void. Buu, shocked that he won't ever eat sweets again, yells and breaks a hole in space-time, escaping into the real world. Buu then kills the remaining Z Warriors. Gotenks transforms into a Super Saiyan 3 and along with Piccolo escapes as well.
Gotenks and Buu fight in the real world until the fusion wears off. Out of nowhere appears Gohan, whose power has increased exponentially thanks to the Elder Kai's training. Gohan (now dubbed Mystic Gohan) fights Buu on equal footing, even after Buu explodes. The allotted time lets Buu formulate a plan, which he successfully uses to absorb Gotenks and Piccolo. Now empowered from Gotenks and intelligent from Piccolo, Buu fights Gohan with a considerable advantage. It isn't until the fused and absorbed Gotenks defuses and Goku arrives once more that Buu is put at a disadvantage.
In a last ditch effort, Buu absorbs Mystic Gohan, reaching unbelievable power. The fight is then halted by Vegeta, who reluctantly agrees to Goku's proposal of using the Potara Earrings to fuse together. The two great warriors agree, forming Vegito. Buu is clobbered, even after turning Vegito into candy. Buu then, in a final move, absorbs Vegito, whose original plan was to save the others trapped inside Buu.
Goku and Vegeta defuse and finally release Goten, Trunks, Gohan, and Piccolo, only to have Buu attack them from the inside. Vegeta discovers the entrapped Fat Buu and rips him out of Super Buu. Barely escaping, Goku and Vegeta witness a regression from Super Buu to a more diminutive Buu, dubbed Kid Buu, the original form of Majin Buu. Kid Buu without hesitation fires a tremendous ball of energy at the Earth. Goku, Vegeta, Hercule, and Dende escape with help from Kibito-Kai, as Buu finally destroys the Earth.
Buu travels throughout the universe destroying planets, until he reaches the Supreme Kai's planet, where he begins his fight against Goku as Super Saiyan 3. Overwhelmed by Buu's original power, Goku is shocked to see Vegeta step in. With Vegeta being pulverized, Hercule tries to find if Fat Buu is still alive. Kid Buu spits up Fat Buu, while later on attacking him.
After receiving energy from everyone on Earth, Goku forms a tremendous Spirit Bomb, which he uses to finally kill Majin Buu. Before Buu's death, Goku wishes that he could fight Buu again, this time with Buu as a good person. Fat Buu continues to live with Hercule as his close friend.
Goku receives his wish at the next World Championship, where a young village boy named Uub appears. He appears to be a strong contender, so Goku leaves the tournament with Uub as his student, with Dragon Ball Z ending after they leave.
In Dragon Ball GT, when Baby attacks, Uub appears to fight. Fat Buu tells Uub of his past as Majin Buu, and allows Uub to absorb him. Uub then attacks Baby Vegeta as Majuub.
Healing: Buu can heal allies using his special magic powers.
Liquidation: Buu can liquify his body and shove it down an opponent's throat. After expanding, the enemy explodes.
Transformation: Buu can transform an enemy into a food, usually sweets.
Human Extinction Attack: Buu fires an energy ball for every person on Earth, nearly eradicating the planet of human kind.
Magic: As seen in the World Martial Arts Championship, Buu can change objects using his magic. He uses this to change the numbers drawn so that Goku can fight Uub.
Regeneration: Being made of energy, Buu can easily reform limbs and body parts.
Death Ball: Buu produces a tremendous ball of energy and uses it to attack an enemy. This was used when Buu destroyed the Earth.
Buu can also learn attacks from others simply by watching them use it. Examples include the Kamehameha, Gallick Gun, Super Ghost Kamikaze Attack, and Instant Transmission.
Kid Buu: Buu's original form is a diminuitive, though nasty opponent. Buu's most vicious and merciless form is equipped with white pants and black armbands.
Second Form: Buu appears in this form briefly during the regression from Super Buu to Kid Buu. In this form, Buu is much more muscular but retains his destructive nature and form.
Fat Buu/Good Buu: Buu's fat form is a playful enemy, but he still loves destruction, at least until Hercule tells him that killing is wrong. Fat Buu(now Good Buu), upon being absorbed by his evil side, is trapped within Super Buu until Vegeta rips him out and Kid Buu regurgitates Fat Buu. Fat Buu (now known as Mr buu) lives with Hercule peacefully, until Baby arrives, where Buu fuses with Uub to become Majuub. Buu retains his white pants, but earns a black vest, yellow gloves, and a purple cape.
Evil Buu: A gray, older-looking Buu who eats Fat Buu. He represents Buu's past destructive nature, which Fat Buu casts off with help from Hercule. He has the same clothing as Fat Buu.
Super Buu: Super Buu appears after Evil Buu eats Fat Buu. His forms change depending on who he absorbs, but these forms collectively are considered as Super Buu.
Uub: Majin Buu's essence was reincarnated into a young village boy. During his fight with Goku, Goku intentionally insults Uub's family, causing his power to be released. Uub then travels with Goku to train.
Majuub: When Uub absorbs Fat Buu, Uub earns Fat Buu's attire and strength, which he uses to fight Baby.