Fusion's are formed when two people do either a dance, or use the Potara earing. The fusion dance is performed many times at the end of DBZ. Mostly by Goten and Trunks. To perform the fusion dance you most abide by these rules:
1. You and your fusion friend (O_O;;) must be standing about 5 feet from eachother.
2.Must be and the same power level.
3. You must know how to do the fusion dance, absolutely perfect or else a misfusion will occur and you will look like crap.
4. You have to be around the same height.
Potara Earings - If you want to use the Potara earing you must follow these rules:
1. Put an earing on the opposite side of the other person.
2. Stand back and wait for a fusion. *A person cannot be brought back, the fusion is permenant. I will explain how Gokuu and Vegeta are put back to normal later on.
Namek Fusion - If you want to fuse with your other NAMEK friend you must follow these rules:
1. Put your hand out in front of the namek you want fused into you. IF you do this you will keep your appearance and not the others.
2. Do something in your body and your namek friend will go into you. This is perminant. You now are fused with your namek friend. I hope you like being stronger!
In Dragonball Z, Goten and Trunks fuse witht he fusion dance to form Gotenks. Their teachers are Gokuu and Piccolo. They are taught the fusion dance and try it after a day. The first 2 times they mess up. The 3rd and 4th times, they do it with perfection. From then on fusing is not even a sweat for these kids.
In Dragonball GT, Gokuu and Vegeta fuse with the fusion dance to form Gogeta. It's pretty cool and they beat the crap out of their enemy. Too bad it doesn't happen more often.
In Dragonball Z, Gokuu and Vegeta fuse with the Potara earings to form Vegetto. Dai Kaioshin gives Gokuu the earings and he was supposed to fuse with Gohan. Gohan gets absorbed, but in the nick of time, Vegeta arrives and they fuse. Vegetto was much more powerful then Buu. If Vegetto had destroyed Buu, their friends would have died. So Vegetto allows himself to get absorbed by Buu. In the proccess the fusion ends and they become Gokuu and Vegeta again.
In Dragonball Z, Kaioshin and Kibit fuse with the earings. Nothing special just thought I should point it out.