What is Fusion?
Fusion, that is an interesting word, and it i used throughout dragonball Z and Dragonball GT. it is in fact used in some of the movies also. It is used in Movie 12 and Movie 13, those are the most popular movies that it is used in, It is used in the series when Gotenks fights Majin Buu, when Vegetto fights Majin Buu, and when Piccolo Fuses with Nail then later fuses with Kami. It is used in Dragonball GT by Goku and Vegeta to form the super warrior Gogeta. Surprisingly gotten and Trunks do not fuse in Dragonball GT. If they did they would become a worthy adversary. Fusion is basically the union of two fighters to form 1 super fighter. It is used mainly because the 2 fighters cannot defeat an enemy on their own, or if they attack together, so they have to fuse into one super-warrior. Below are some of the different types of fusion used throughout the series and in the movies.

Porta Earrings
This fusion is the strongest fusion out of the three mentioned in this info page, this fusion involves 2 individuals, but after they are fused they can fuse again with another person, and so on and so forth. basically this is an infinite fusion between 2 people. The only bad thing about it is that which i have said, because they will never be able to separate. But once they are fused their power level goes up dramatically, and they become almost invincible. The funny thing about this is that it can work against you also, meaning that if you fuse with someone really weak, then it will hinder your power. For example when the Elder Kai fused with the witch. But other then that this fusion is limitless because as said above, you can fuse as many times after you have fused, so if you don't understand, I can explain it a little more. Goku and Vegeta fuse using this technique to make Vegetto a super strong warrior, then Vegetto Fuses with Gohan, to make an even stronger warrior, and then After that whatever the product of Gohan and Vegetto is they can fuse with someone else that is really strong like trunks or gotten, and make some unimaginable Deity of pure power, so you see this is quite a good way to fuse, but you have to be careful, and you have to consider the risks.

Namekian
Namekian Fusion is a unique fusion which is only limited to nameks. It can be used either to separate 1 namek into 2 separate nameks, like Kami and Piccolo, however, these two nameks will still rely off of each other for life, meaning that if one dies the other will also die. This fusion can also be like the Porta fusion, which just fuses 2 nameks together, this fusion only works on Nameks, and no other race. It does not require any sort of earrings, or devices, all they have to do is touch each other and concentrate. Once fused they will become a super warrior consisting of both the fighter's power levels put together. As with the Porta fusion, this type of fusion is similar in that the fighter is able to fuse again after he has fused once. For example when Piccolo fused with Nail, and then Went on to Fuse with Kami. This fusion is considered to be permanent, however it is not for sure, because Kami did separate himself into 2 parts, which only means that piccolo can once again separate himself into 2 parts. But it is never shown in the series, so we can only assume.

Dance
The Fusion dance, probably the most inconvenient, and weakest way to fuse, because it only lasts for 30 minutes, which could be either a good or bad thing depending on how you look at it. It could be a good thing because if you don't want to be stuck in the same body as your rival (Vegeta and Goku) then all you have to do is fuse and defeat your enemy, then 30 minutes later you un fuse. The bad side of it is that if you don't have enough time, meaning that the enemy is so strong that you need more then 30 minutes to defeat him or her. When you un fuse you are vulnerable, since you can't fuse again for a while, because you have to let the power recharge. In the example of Gotenks and Majin Buu, Gotenks didn't have enough time to defeat Majin Buu so he un fused, and would have got thrashed if Gohan didn't show up. This fusion involves the 2 people to do a series of maneuvers, they both have to be exact, and when done right the fusion will be successful. If one of the parties makes the slightest mistake in the angles of their feet, fingers or any other body part, then the fusion will either not work, or it will be messed up. (Fat Gogeta) (Fat Gotenks)