The Fusion Dance
First their is the most used fusion, the fusion by way of the Fusion Dance. To execute this fusion correctly you must have someone to fuse with that is about the same height, weight, power level and you must do the dance correctly. If the dance is done incorrect then it will result in in some sort of mutation, but if executed correctly you have one powerful warrior. After doing the fusion dance you will notice that the incredible increase in strength, speed, and vitality. This fusion only last for about a half hour. This can be both bad and good. It's bad if you're in the middle of a fight and you're kicking some butt when all the sudden you unfuse into two little weaklings. It can have an upside too. I can't really explain but it came in handy when Buu absorbed Gotenks and then they unfused making Buu weaker.
Who has used this fusion? The first two people to do this fusion were Goten and Trunks to become Gotenks. Gotenks is a cocky little guy, capable of reaching SSJ3, and beating just about anybody he fights. Goku and Vegeta also had to fuse into Gogeta to beat Janenba in movie 12. Gogeta is an awesomely strong warrior, and does what not even Goku could do as a level 2 Super-Saiya-Jin, and that's beat Janenba.
Fusion through Potorra Earrings
The second fusion is fusion through Potorra Earrings. Potorra Earrings, are earrings that all Kaioh-Jihns wear. If you have one person pop a Potorra into their left ear and another pop it into their right ear, you have the ultimate fuision. The fusion has an upside and a downside. The downside is that you have to stay fused together forever, the fusion is permenant. The upside is that when you fuse, you will be one of the strongest warriors in the universe.
Who has used this fusion?
The first person to use this fusion was the Elder Kaioshin, and a witch. She supposedly takes the earing out of his ear and says "You're a boy. What are you doing wearing earrings? Here, let me try them on they will look good on a beautiful woman like me!" (She was a very ugly witch). The second person to use this fusion was Kaioshin and Kibit. They fused right before the fight with Buu, but even with their combined strength they werent enough to even have the chance to fight Buu. The third and last two people to use this fusion were Goku and Vegeta, who fused to become Vegetto. Vegetto is the strongest person in DBZ. He easily beats the crap out of Buu, but because he wants to rescue his family and friends he lets Buu absorb him. Unlike usual the Potorra's broke, the fusion wore off and Goku and Vegeta were seperated again.
The Namekian Fusion
The Namekian fusion can only be done by Nameks. This fusion is permanent and comes in handy in the fight with Freeza. If Piccolo had not not fused with Nail he wouldn't have stood a chance with Freeza. Not only did it make him a lot stronger, but also gave him the advantage of more knowledge, and knowing all of Freeza's moves. This fusion has an upside and a downside. The downside is that it is permanent and will never wear off. The upside is that it will make you a lot stronger, both mentally and physically.
Who has used this Fusion? The only three Nameks to use this fusion were Piccolo, Nail and Kami. First Piccolo fused with Nail after being persuaded that it would be impossible for him to beat Freeza with just his strength alone. The second and last time this fusion was used was Piccolo, already with Nail, fusing with Kami in order to help in the fight with Cell. This is by far my least favorite fusion. Why? The characters appearance doesn't change at all, and it looks stupid executing the fusion.
