Rape is the act of forcing someone to engage in sexual intercourse against their will. Statutory rape is engaging in sexual intercourse with someone under the legal age of consent.
Disturbing numbers of badficcers seem to think that rape is a trivial matter that can be used to bring hurt/comfort, and forget the deep psychological impact that being raped has on the victim. Failing this, they seem to think that repeated instances of rape will end in the victim willingly participating and indeed falling in love with the rapist. Technically, what Mary Sues do when having sex with their canonical lust objects falls under the heading of rape, as the LO is usually so mind-controlled that they are not capable of giving informed consent.
Occasionally, fic involving rape can be good. "Vengeance Quest," for example, mentioned it a lot and featured it happening on at least three occasions, but is still considered a goodfic. This is partly because the rape scenes were not graphic and therefore significantly reduced in squick, but mostly because there was a logical reason for them to happen and the characters reacted in a realistic way to them happening. This does not comfort Agent Stormsong, who was going to be the victim of an unpleasant gang-rape before his rescue and recruitment, and now suffers graphic hallucinations of said occurrence if shocked or excessively squicked.
Agents Laburnum and Foxglove have been sent to FicPsych and disciplined for allowing their targets to suffer rape on two separate occasions; once when they gave Raketooth and his companion to the Redneck Trees of Something Positive fame, and once when they gave Jackson Earl Simon to Drip Rat. Furthermore, Agent Rez got in trouble for giving a River Tam character replacement to the Reavers. This is one area in which the PPC thinks that poetic justice can be taken too far.
Not to be confused with BDSM, in which the point is that everyone involved agrees to whatever's being done, however gruesome it may seem to someone who's not into it.