From Dragonball Online Wikipedia
Saiyans are a very powerful race of people. Some Saiyans are equipped with greater powers that make them stronger than the others, and can undergo special energy transformations and take the Super Saiyan form. The transformation stage doesn't stop there. There are other transformations that allow the Saiyan to exceed the Super Saiyan form, and achieve up to four different levels of the form. Only six Saiyans; Bradley, Goku, Vegeta, Trunks, Goten, and Gohan can perform transformations called a fusion. In most fusions, there's only one form that the outcome can take, however, Goku and Vegeta can fuse to become either Gogeta or Vegeto. Sayians also become more powerful if they are defeated in battle and survive.
Goku and Vegeta Fusion: Gogeta or Vegito
Goten and Trunks Fusion: Gotenks
Bradley and Future Trunks Fusion: Brunks