Gotenks and Gogeta (also Vegetto)
Gotenks is a form of both Trunks and Goten. Gotenks is formed when Trunks and Goten, both performs the fusion dance correctly. That means they have to be side by side in almost perfect alignment. After that they start the dance and shout, "Fusion Ha!" If they are not in perfect alignment, they hands or legs out of place, the fusion would most likely backfire. This is exactly what happens to Vegeta and Gokou when they tried the fusion dance for the first time. Since Vegeta thought doing this "dance" was stupid, he accidently had a finger or hand out of place, and they ended up fusing into a fat Gogeta instead, of a normal sized one.

The benefit of fusion is that, ultimately you end up with a stronger being than before. This is so, because of the combine power and ki, that the fusion "being" possess. One thing though for the fusion to work is that, both Goten and Trunks have to possess similar power levels. If one person is significantly stronger than the other, the fusion dance will not work. Since both Goten and Trunks had similar fighting experiences, they have a good chance of successfully performing a fusion dance. Vegeta and Gokou's power levels, although different are able to perform the dance. Since Gokou is able to lower his ki, and raise it, Gokou would most likely change it accordingly to how strong Vegeta is.

Another way of fusion is by using the "Pottara Earrings". Both characters most wear an earring, on opposites ears. So if Gokou wears an earring on his left ear, Vegeta must wear the other on his right ear. After that they fuse together. When Gokou and Vegeta fuse with the earrings, they become "Vegetto". Not to confuse you but, when Gokou and Gogeta use the dance they become Gogeta, and when they use the earrings they are Vegetto. Only Gokou and Vegetto had fused using the earrings, Goten and Trunks, or any other people have not used the earrings. Except for KaiOhShin, who gave Gokou and Vegeta the earrings in the first place, who used the earrings a long time ago. Using the earrings, has its disadvantage. When using the earrings, the fusion is supposedly "permanant". So how did Gokou and Vegeta separated, if they are the only ones that use the earrings? Well what happened in the Buu Saga, when they first use the earrings, got swallowed by Buu himself, when they were fused. While in this "magical being's stomach" somehow disable the earrings' effects, and they separated.

Another form of fusion is the "Namekian fusion". You see the first evidence of two nameks fusing in the Frieza saga, when Nail and Piccolo fuses. But unlike the previous methods, there are no earrings that needs to be worn or a special dance that needs to be performed. If the two beings fusion, are both Namekian, then they are able to fuse together naturally. The product of this fusion, is of course one stronger being. This fusion, like the earrings are permanant. Also the dominant namekian (whoever it may be, in DBZ it's Piccolo), is not rid of the "other" namekian. So Nail and Piccolo are able to communicate, even though they are both one being. Also later in the android saga, Piccolo fuses with Kami-sama, because he realizes he will not be able to defeat the androids, without his help. Kami sama does so because he himself does not have the power to defeat the androids, and he doubts even Gokou will defeat them. They also decides that Piccolo become the "pillar" the dominant of the two. Piccolo would have to touch Kami-sama's chest and the two would start to fuse. This leads to a dramatic moment, and plays a lot into the Cell Saga. (warning: Kids, do not try this "fusion" technique on girls. You'll probably have a slap comin' yo' way..) So, Piccolo and Kami-sama once again reunite, into one Namek-sei-jin.

Fusion Dance Guidelines:

1.) Must perform fusion dance correctly
2.) Must have similar power levels
3.) Fusion dance is not permanant, it wears off in 30 minutes
4.) If both are in a SSJ state (SSJ1, SSJ2, etc.) when they fuse they become 'SSJ Gotenks or SSJ Gogeta". For example, SSJ2Gokou + SSJ2Vegeta = SSJ2Gogeta

Pottara Earrings Guidelines:

1.) Both must wear the earrings on opposite ears
2.) After they wear the earrings, they "fuse"
3.) Fusion is supposedly permanant
4.) In DBZ only Vegeta and Gokou has tried fusion using only the earrings (Besides Kaio-shin)

Namekian Fusion Guidelines 1.) The two beings fusing, must be two Nameks
2.) Once they fuse they are permanantly fused together.
3.) The two or three beings are able to communicate with each other, if they want
4.) Like all fusion, the results is one stronger being