Chibi Trunks + Goten = Gotenks
Fusions
fu·sion (few-shun)
n. The act of combining; merging.
During DragonBall Z/GT, desperate times called for desperate measures and "fusion" was one of those measures. In the show it's when two (can be more for a Namek-jin fusion) characters combine to form a much more powerful being. Usually one being is dominant and his or her personality is more apparent but the fused character's overall personality is completely unique. Other then the Namek-jin fusions, fused characters are usually extremely cocky. Everything else varies from the characters who fused and what method(s) they used to do it. Listed below are the types of fusions (successful, and unsuccessful/imagined) that occur in DBZ and GT.
[The Many Types of Fusion]
Fusion Dance: For a successful fusion, two characters of about the same race, species, height, weight and power level do a complicated dance. If there are no errors, they "fuse" -- or merge -- and become a singular being of extravagant power. Should the dance not be performed correctly, the being will be extremely weak. This type of fusion wears off in 30 minutes.
Chibi Trunks + Goten = Gotenks
Potara Earrings Fusion: It has the same results as the Fusion Dance, the characters must simply wear the Potara Earrings and look at each other to fuse. However, there's one "slight" downfall: the users stay fused forever. In the Buu saga Goku and Vegeta are able to defuse back to their original selves after entering Kid Buu because the laws of physics differ within Buu.
Namekian Fusion: Used by only the Namek-Jin, this fusion
is basically the absorbtion of another Namek. The lead of the fusion places
their hands on the other Namek's chest, and "absorbs" them, gaining both their
knowledge and their power. This fusion is permanent and the characters cannot
defuse. This is why Kami-sama took the time he did he before deciding to fuse
with Piccolo.
[The Successes]
[The Fake or Unsuccessful Fusions]