Fuses..........

Fusion Dance:

By doing the Fusion Dance, two fighters of the same level can become one fighter.
There is a time limit of 30 minutes, after that time they turn back, and have to wait 60 minutes before they can do the fusion again.
This fusion has some disadvantages. If you mess up the dance even the slightest bit, you can end up being a very heavy fusion, or a very old one.

People who fused like this:

Trunks - Goten = Gotenks (Dragonball Z)
Goku - Vegeta = Gogeta (Dragonball GT)

Potaro Earring Fusion :

This fusion is a lot better than the Fusion Dance, because the fusion is stronger and there is no time limit.
The two fighters who want to fuse have to wear Potaro earrings. One fighter wears a Potarro earring in his left ear, the other in his right. At that moment, they will me merged into a single fighter. Normally this fusion is permanent..
Only Goku and Vegeta were able to split up after this fusion, when they were swallowed by Majin Buu.

People who fused like this:
Goku - Vegeta = Vegito (Dragonball Z)
Shin aka Supreme Kai - Kabito = Kibito Kai (Dragonball Z)

Namekian Fusion :

Only Nameks can perform this type of fusion. After being fused, there is no turning back, ever.
The Namek who remains keeps his own body, but the other Namek's wisdom and powers are absorbed.

People who fused like this:
Piccolo - Nail = Piccolo (Dragonball Z)
Piccolo - Kami = Piccolo (Dragonball Z)