The Fusion Dance:
The Fusion Dance is a way that two people use to fuse together, if the fusion Dance is done correctly then you'll get an almost perfect being who's power is unmatchable, if the fusion dance goes wrong then you'll end up with a being who is deformed quite badly, the most common defromaties are:
1: The being is so fat that it finds motion difficult (e.g. Fat Gogeta in movie 12)
2: The being is so thin and brittle that it would get beaten very easily in any battle
Only two pairs of people use the fusion dance in Dragonball Z:
Goku and Vegeta = Gogeta (e.g. Movie 12)
Goten and Trunks = Gotenks (e.g. In the Majin Buu saga)
When you fuse using the Fusion dance the Fusion only lasts for 1/2hr.
The Potara Earrings Fusion:
After Goku trained Gohan with the Z-Sword (In the Majin Buu saga) Kaioshin gave him a pair of earrings, The Potara earrings. The fusion lasts forever and is supposed to bond the peoples looks, hairstyles, and attitudes into one, supposedly this makes an incredibly powerful being. The Potara Earrings Create a much more powerful being than the Fusion Dance can create. To make the fusion work:
1: You both must have an almost Identical power level.
2: The earrings must be put on opposite ears to each other.
3: You may only attempt the Fusion ONCE in your life, it cannot be done twice.
Only one pair use the Potara earrings to fuse:
Goku and Vegeta = Vegetto (e.g. In the Majin Buu saga)