Occasionally, powering up to extremely high levels seems to somehow affect gravity. Clouds and waves may form, or craters around the powerful person. A force or intense wind is also present when charging. It is strong enough to blow people off their feet in some occasions. Sometimes the pupils in the eyes of a Saiyan temporarily disappear, although I've only seen it happen once or twice.
Fusion is when two people are combined into one different person.
3 methods are used in DBZ, although only 2 are commonly reffered to. Only Nameks can do this type. Nothing specific is done, they just stand next to each other and yell. Piccolo also fused with Kami this way. The power levels of the people have to be the same, but that isn't a problem since Saiyans can change their levels.