The following are the 3 types of fusion there is in Dragonball Z/GT.

Fusion Dance: This fusion is done when the 2 characters perform a dance (which is a mirror of what the other character does) to fuse into a whole new character. The Characters must be around the same height, build, and power level. (The person with the higher power level can lower his level to the other characters power level) This fusion lasts for exactly 30 minutes. After this period, the characters must defuse and have a 1 hour break before fusing again. Goku learns this dance while is was up in heaven between the Cell game and Gohan's first year at Orange Star High School. There are only 4 characters that can fuse like this. Goku and Vegeta fuse to form Gogeta, and Trunks and Goten fuse to become Gotenks. Gogeta is the strongest character in DBGT.

Potara Fusion: This fusion is done with earrings called "Potara Earrings" This fusion is supposed to be way better than the fusion dance. This fusion was originally used by kaioushins. The two characters that wish to fuse must wear an earring (wearing it on the opposite side of the other character) After putting on the earrings, the characters can now fuse into the new character. This fusion is supposed to be a permanent fusion, but the character defuses because of an unexpected condition. Each character can only do this fusion once in their entire life! Vegeta and Goku use this fusion to fuse into Vegetto. ***Note Goku was originally supposed to fuse with Gohan, but Gohan got absorbed so Goku had to fuse with Vegeta.***

Namekian Fusion: This Fusion is done by Nameks only. There is no special dance needed to perform a namekian fusion. All that needs to be done is One namek puts his hand on the other namek, and they fuse to become on being. depending on the character you fuse with, you will be able to defuse, while other times, you wont. Piccolo and Nail fuse to in the freeza saga (they are allowed to defuse) and Piccolo and Kame also fuse (they cannot defuse)