Fusion Guide

Gogeta:
This is the fusion between Goku and Vegeta. We see Gogeta in DBZ Movie #12: The Rebirth of Fusion!! Goku and Vegeta and again at the end of Dragonball GT when Vegeta and Goku are trying to stop Omega Shenron.

 
Gotenks:The fusion result of Goten and Trunks. Gotenks is seen in the Buu Saga when all else has failed to stop the evil Buu. It takes Goten and Trunks awhile to finally get the fusion right and when they do get it right they mess around to much and end up running through their timelimit.

 
Kamicollo:This is the fusion between Piccolo and Kami. Since they were once a single being, rejoining the 2 halfs made them much stronger than if one of them fused with someone else. Before the 2 fused they decided on keeping Piccolo's body since it was much stronger & younger.  

Kibito Kai:This fusion wasn't intended and happened accidentely when Supreme Kai and Kibito were showing Goku the Potara Earrings.  

Old Kai:This fusion happened when an old witch grabbed the Potara Earring off of the Supreme Kai 15 generations back. When the witch put on the earring the two were stuck together forever. 

Piccolo:When Piccolo was wished back to life on Planet Namek he quickly picks up Frieza's power and starts to travel to it. On the way he comes accross one of the last Namekians, Nail. Piccolo is informed that he is no match for Frieza but if the two fuse they just might stand a chance. Piccolo is hesitant but finally agrees. They decide on keeping Piccolo's body and when the fusion is over Piccolo goes on and on about how great he feels.

Super 17:
When Dr. Gero and Dr. Myuu meat up in hell they make a new Android 17. When this new Android 17 escapes from hell he meets up with the old Android 17 that Gero had made years back. The two fuse and create the all powerful Super 17.

Vegito:We see Vegito when Goku and Vegeta fuse using the Potara Earrings. They would never have fused knowing that they could never again seperate but they knew they had to in order to defeat Majin Buu. When Buu absorbed Vegito they happened to defuse. Vegeta and Goku then crushed the Potara Earrings.