Goten and Trunks form this new entity by the Metamoru art of Fusion, which was taught to Goku and is passed on to Goten and Trunks when Goku comes back from the Other World. The fusion must occur between two beings of equal power (Trunks has to lower his power level to fuse with Goten), and is accomplished by a goofy looking, mirror-image dance. This type of fusion only lasts 30 minutes, and they must recuperate for one hour before fusing again.
Goten and Trunks are first trained in the art of Fusion by Goku and then by Piccolo. They try several times before getting it right, creating two awkwardly formed Gotenks in the beginning. Once perfected, the fused Gotenks is an incredibly powerful fighter. Like Goten and Trunks, Gotenks has an amazing ability to learn moves quickly, and quickly manages to go to Super Saiyan and even Super Saiyan 3 or third stage Super Saiyan after watching Goku do it once.