Ratings
G= Very clean, as in no strong language, no violence, and nothing beyond a very innocent kiss or two.
PG= Still pretty clean, with nothing past kissing and implied sex, but it can contain mild violence or the occasional swear word.
PG13= Sexual thoughts and making out is okay, but no actual sex scenes. Can also contain strong language and non-sexual violence.
R= This is a good rating for fic in which there are full sex scenes, but they're not explicitly detailed. Almost all levels of violence and language can be covered by an R rating.
NC17= The equivalent of the old 'X' rating.
Warnings
Non-Consensual Sexual Situations: If it's not rape, but it's not really clear that both characters want it. It also applies to rape situations where the attack is cut short, and not completed.
Rape: Consummated sex that is fully against one character's will.
Incest: If the characters are closely related, such as father/son or brother/brother, this warning will appear. If they're cousins or less, it may or may not be (it depends on if the author chooses to include it or not).
BDSM: Covers bondage/discipline and sadomasochism. Generally it implies that the sex is consensual, though, unless there is a non-con warning as well.
Graphic Violence: The violence in the fic is very intense, explicit and disturbing.
MPreg: Yes, it means "male pregnancy." Such stories are may appear here; it's best to be warned ahead of time so you don't get too confused.
(I stole this from The Library of Moria)