Types Of Fusions
In this type of fusion two characters of around the same height and power do a special dance turning them into one person. The dance must be done perfectly or it will not work right. This fusion lasts for around 30 minutes.
Potara Earring Fusion
In this fusion two characters take one of the Potara Earrings from the Kaioshin's and puts it on the ear of the opposite person. This fusion is suppused to be forever but in Buu it does no apply.
Two Namekjins get together and place each others hand on the person who they are fusing with's chest. The fusion can last forever. Some Namekjins split into to beings to get rid of there evil but this is not a fusion.
Cell absorbed 17 and 18 with his tail to become perfect. There were also other android fusions that are like Namek fusions.
The fusion of Chibi Trunks and Goten which is acheived by yhe fusion dance. He is the only character besides Goku that reached the SSJ3. He is a very powerful fighter but wastes time because he is so young and cocky.
This is the fusion between Goku and Vegeta using the Potara Earrings. Vegitto fights Buu to try and stop Buu from destroying the world. He is probably the most powerful fighter in all od dbz.
This is the resault of Vegita and Goku after preforming the fusion dance. We see Gogita on two occasions probably the most memorable one was in the battle against Janemba.
Piccolo fused with two Namekjins, the first being Nails while on Planet Namek, and the second being with Kame to become a "Super Namek" in the fight against Cell.
This is Cell's final stage which is acheieved when he absorbs 17 and 18. In this stage he is very powerful and is only beaten because of Gohan's transformation into a SSJ2.
This is what happens when Dr Gero's machine builds 3 new androids from Dt. Gero's hatred. 13 is designed to kill Goku and after the deaths of 14 and 15 he assembles there parts to himself. He becomes Super 13 and is in the end defeated by Goku in movie 7.
Super 17 is the merging of Dr. Myuu's replica of 17 and Dr. Gero's original 17. Together they make a powerful android who appears in GT.
Rou Dai Kaioshin
He was the one tapped in the zed sword.He fused with a witch(see USELESS INFORMATION).