FUSION

"I'm not Goku or Vegeta , I'm Gogeta. It's over Janaepa. I've come for you"

                                                                                                              Gogeta in Fusion Reborn

 The fusion technique was developed by a race of aliens in the other world where Goku learnt it.                                                                                                      
The fusion technique is the technique in DragonBall Z and GT where 2 warriors combine bodies by doing the fusion dance
properly and results in increased strength and creation of a powerful new being . The fusion technique lasts only 30 minutes after which the warriors seperate. After the fusion was succesful and has expired, the warriors may fuse again after 1 hour only. However if the fusion is unsuccesful the warriors may fuse succesfully before that.

Known users  and their succesful fuses.:-                                                                                                

Goten and Trunks - Gotenks
Goku and Vegeta - Gogeta (In Fusion Reborm and DragonBallZ-GT)
Yamcha and Tien - Tiencha
Failed Fusions:-

Goku and Vegeta - Veku( weak version formed when Vegeta performs the fusion dance improperly)
Veku is seen in Fusion Reborn                                                                  
Goten and Trunks - Fat Gotenks ( Dragon Ball Z, Dragon Ball Z- Buu's Fury, Yo! Son Goku and his friends return)
Goten and Trunx - Old Gotenks (lack of coordination) (Dragon Ball Z, Dragon Ball Z - Buus Fury)
Yamcha and Tien -  Skinny tiencha (Dragon Ball Z: Budokai2)

All fusion technique fuses wear the same outfit.The one seen in Gogeta's pic.Gotenks wears a smaller one.

For the fusion to be succesful the warriors must be of the same power level and must be about the same height. Most importantly, They must perform the fusion dance properly.

And a fun fact -  You may notice that gogeta and gotunx look similar in appearance. Why? Goku and Goten and Vegeta and Trunx are father and son.