I've seen the term "lemon" lots in the HP fandom, but it refers to a specific erotic scene or chapter. As in, "Sorry no lemons in this chapter, but they're coming soon!".

Slash, the way I've heard it used, only refers to pairings of gay males. There's "femmeslash" for two women, or "het" for male/female pairings, though I don't see that one very often anymore. And a "ship" is the particular pairing you like, short for "relationship". As in, April ships Harry/Draco, and I ship Snape/Hermione. Those are the most common fanfic terms I've seen used.