Gotenks is the cocky result of fusion between Son Goten and Trunks. Gokou, with the help of Piccolo, taught them the Fusion dance in hope that they could defeat Fat Majin Buu. The two boys were quite inattentive and even went so far as to insult Gokou's power, until they witnessed Super Saiyajin 3 Gokou. Needless to say, they sharpened up real quick. Their first two attempts were complete failures and when they finally preformed the dance correctly, they challenged Majin Buu without even going Super Saiyajin (Hey, I said he was cocky). When the boys wise up and realize they can't beat Majin Buu yet, Piccolo sends them to the Spirit and Time Room to train. Piccolo also has some sorta mindset that Gotenks's name is really Trunkten, that was mad funny...

Gotenks is incredibly strong and once he turns Super Saiyajin 3, he could have easily defeated Buu. But, like all fusions, he's too sure of himself and makes stupid mistakes. Hey, did you really think Toriyama would end Dragon Ball Z that way while Gohan is getting trained on Kaioshin's planet?

The next time he appears is in the last few episodes of DBZ, when they're fooling around with Gokou. But Gokou shows off by going SSJ.

Gotenks also appears in DBZ movies #12 & 13. In movie #12, Goten and Trunks sense Gojita's extreme power level on Earth (Keep in mind Gojita was in Hell, that's how powerful he is) and decide to fuse, too. They use the Super Ghost Kamikaze Attack to destory the reincarnated Hitler and his army (Very funny). In Movie #13, Gotenks appears at SSJ3. Although he wass powerful, he still wasn't strong enough to defeat the threat.