Fat Buu
This is the first form we see be in, but not actually his first form. When he manages to absorb the Grand Supreme Kai in Otherworld, he takes on not just a similar appearence, but a love for sweets, and more pure of heart. Later when Evil Buu is seperated from him, Fat Buu becomes pure good and actually joins up with the good guys.

Evil Buu
This is the pure evil side of Fat Buu seperated out of his body. He is much stronger than Buu, and actually manages to absorb his good half, and become the all powerful Super Buu.

Super Buu
After Evil Buu eats Fat Buu in cookie form, he transforms into Super Buu. Super Buu gets a power increase and seems to be a lot more intelligent and talkative than his previous forms were.

Super Buu (W/ Piccolo)
When Super Buu absorbs Piccolo, his intelligence rises even further than it had before. He gains all of Piccolo's attacks, too.

Super Buu (W/ Gotenks)
Super Buu cleverly absorbs Gotenks by releasing Buu Goo from his body and surrounding Gotenks with it, thus bringing him into his own body bycontrolling the goo. He gains some of Gotenks powers as well as a longer head piece.

Super Buu (W/ Gohan)
After absorbing Gohan, Super Buu turns into these even stronger form. At this point, it seems the Potara fusion of Vegeta and Goku is the only thing stopping him.

Kid Buu
After Goku and Vegeta rescued everyone who was inside Buu, he turns into his true form... Kid Buu! This form isn't as talkative as the others, but he packs quite a wallop. He can even learn attacks just by seeing them in person.

Super Kid Buu
In a flashback we see in the series, Super Kid Buu is shown after Kid Buu absorbs South Kaioshin. He kills two other Kaioshin (North and West), East Kaioshin is able to escape before being killed. He doesn't stop there however, he absorbs Dai Kaioshin which is what turned him into Fat Buu.

Uub
After Kid Buu's defeat Goku makes a wish to have him rencarnated as a good person. Uub (Buu spelt backwards) is the result of this wish. We see him fight Goku on the very last episode of DBZ they then go off to Kami's Lookout and train for 10 years.

Majuub
In DBGT Uub fuses with Fat Buu and gains his ability to turns anything into chocolate along with Fat Buu's other moves of course. He fires his turn into chocolate move at Baby Vegeta and lets Baby turn the move back on Majuub turning him into chocolate instead. Little did Baby know Majuub is strong enough to reverse this and starts powering up inside Baby!