Fusion is a technique that can be used to fuse fighter's bodies power into one. There are multiple methods of fusion that range from temporary to permanent.
 Fusion Methods
 Namekian Fusion
As shown by Piccolo and Nail, some Namekian's also possess the ability to fuse and combine their powers into one body. This type of fusion would seem permanent, as Nail never left Piccolo. Kami split into two, but that was merely a split of good and evil, not two actual Namekians. After Piccolo's fusion with Nail, he later the re-merged with Kami. Three spirits, three bodies, connected as one. Piccolo and Kami originally being one however made the transition easier as technically, it was still only a fusion of two seperate Namekians even after Kami and Piccolo's re-merging.
 Fusion Dance
The fusion dance technique was taught to Goku by the Metamorese race. This method of fusion is temporary, and only lasts for 30 minutes. Inside the Room of Spirit and Time however, the time limit for fusion is increased as is time in general. This method of fusion was only used by the Saiyans.
 Potara Fusion
This method of fusion is brought on by two people putting on a pair of Potara Earrings that are in the possession of the Kaioshin's. One puts the left earring on their left ear, the other puts the right earring on their right ear, they come together and merge into one being. This type of fusion is meant to be permanent, but Goku and Vegeta were magically seperated from being Vegito once they entered Majin Buu's body.