I stole this definition from Pointy Stakes because I'm lazy and it was already there.
SLASH describes a specific type of romantic/sexy fan fiction -- stories in which the couple is male/male, or female/female. Those are usually considered "unconventional relationship" stories because most tv shows don't have gay couples on them -- though there are some, like Buffy's Willow and Tara.
The word "slash" originally comes from the shorthand that fan fiction writers used to describe who the members of the romantic pair are: "name/name". On the surface, this doesn't make a lot of sense, if you've read much fanfiction, because most writers use that "/" symbol for boy/girl pairings, too, right? Like "Buffy/Angel." "Lana/Clark." Watch me shrug. Yes, it doesn't make much sense, you're right. The legends say, the first use of the "/" was to describe male/male couples, so the word "slash" became associated with male/male, and later with female/female, as well. Not having been around back then, I can only take people's word for it. Simple answer: when someone spells out the word "slash," you know it's a same-gender couple.
RATINGS
G - Fun for the whole family. Kids can look at the original story, but I still wouldn't recommend reading the M/T.
Twist: Well, yeah, cuz you've got that potty-mouth.
Sister: Yes. Moving on.
PG - It's a little more risque than G. There's probably kissing. But again, the M/T is probably worse.
PG-13 - Slightly worse. Kissing with tongues.
R - This is where it gets fun. Groping, grinding, coming...it all happens, but it's not really in detail. But still, it's there, or implied.
Twist: Yay! Sex!
PHT: Didn't you hear her? She said implied.
Twist: Well, crap.
NC-17 - Heeeeere we go. This is the good stuff. This stuff has sex. Detailed sex. Unfortunately, in bad slash, this isn't always a good thing.
Twist: Yeah, we had to read Goldibuffy.
All: *shudder*
Sister: So, that's all you need to know. Well, for now, till Twist or the Pink Headed Troll tell me otherwise. But for now, this is all you're getting. So suck it up.
Twist: Do you always have to be such a bitch?
Sister: Yes.
