1. A type of "what if" fanfic where one (or more) of the characters is the opposite gender from canon but otherwise has the same personality and back-story. This can either happen as a part of the plot, or simply be a given. Most often it is a male character that is turned female, in which he is referred to as femme!character or girl!character. The female equivalent is male!character.
2. Fanart where the character is the opposite gender from canon.
3. Partial genderswitch happens almost exclusively to male characters, when they, as part of the plot, gain some female parts (boobs, vagina, womb) but also retain some male parts (penis, testicles).
4. One of the devices used to make M-preg somewhat plausible.
5. A hell of a lot of fun to write.
I just wrote this great genderswitch fic where Voldimort turned Draco into a girl to punish him and then Harry knocks him... er... her up.
I drew genderswitched Ed Elric, because he's pretty enough to be a girl so why not take it the rest of the way.