I still dont get WTF shipping actually is.
A form of fanfiction pairing up various characters romantically/erotically, often characters who would not be paired in such a fashion in-universe.
Also known as "slashfic" from the practice of referring to a given "ship" by the two characters' names seperated by a slash. Such as "Buffy/Spike".
Slashfic actually has pretty specific connotations nowadays, and is generally reserved for stories involving male homosexuality as a primary story point. Femslash is the female variant. You'd be looking at something like Spike/Angel(us) in that continuity, if it were a slashfic

S'generally just shortened to slash, though. More or less an equivalent to yaoi or shonen-ai.
First bit's accurate, though! Though it's less a form of fanfiction so much as a general aspect of fanfiction. It gets involved in stuff that doesn't have much of a focus on relationships, et al. S'mostly just shorthand for a "these are major characters and who they're primarily interacting with" sort of thing.
The more you know! *rainbow*