Basic Trope: An unnecessary rape scene is shoehorned into a work with little to no relevance to the plot, often due to bad writing.
Straight: Bob rapes Alice for seemingly no reason. It has nothing to do with the story, and is never mentioned again.
Bob's personality does a complete 180 and he rapes Alice for no reason. He also forces himself upon Carol, Ted, Fido, and anybody else in the vicinity. After he's had his way with Anything That Moves, he goes after inanimate objects.
The original plot is forgotten about and becomes nothing but Bob raping Alice over and over again.
Downplayed: Bob does not sexually assault Alice, but he does something that symbolizes rape, such as stealing something from her and bragging about it. Despite this, there was still no reason for him to do it.
Justified: Bob lives in a socially backward society which doesn't look on rape as being especially evil.
The rape scene was written first, and an unrelated plot was slapped on.
Zig Zagged: Bob captures Alice and pins her to the ground, but then kills her by stabbing her. Bob then captures Ted and pins him to the ground too, only to threaten to gouge his eyes out if he doesn't tell him where the Unobtainium is.
Bob just kills Alice.
Bob uses non-sexual methods to get information out of Ted, such as waterboarding.