Fusion is a unique power introduced in Dragon Ball Z where two or more people can combine to create a stronger being. Various methods have been employed to achieve the same end-result.
Namekian Fusion is the first type of fusion to appear in canon. Akin to absorption / assimilation, one user absorbs the entire essence of another namekian, thus achieving all of his power, knowledge, and techniques in the process. So far, there is no known way to cancel this fusion.
Gast Carcolh is a primary user of this type of fusion, having inside of him the incorporated powers of Nail, Elder Guru, and several other Nameks.
Metamorese Fusion Dance is a powerful fusion method that combines two users into one.
Gokū and Vegeta also used this technique to create a fusion; an immensely powerful fusion named Gogeta.
Potara Fusion is a way to fuse using the Supreme Kais Potara earrings. Unlike other fusions, Potara Fusions last forever.
Vegetto of Universe 16 was born from desperation of defeating Buu. Vegetto would remain fused in the events of DBM.
- It was revealed in Dragon Ball Super that the Potara Fusion only lasts an hour for beings who are not Kais, but this is not present in DBM, due to DBM being created several years before DBS' creation.