Definitions
Yaoi: Refers to male homosexual relationships. So boys will be kissing boys (or at least wanting to). Similar terms not used on this site: shounen ai (literally "boy love"), m/m, slash.
Yuri: Refers to female homesexual relationships.
Lemon: Explicit sex scene. (From an anime called "Cream Lemon" or something similar.) Please don't read these if you are underage.
Lime: Not a full-blown lemon, but still racy.
PWP: "Plot? What Plot?" - A plotless lemon.
Alternate Universe (AU): I use this when all the characters in a crossover have very different backgrounds than they do in the original series. So, if in a Gundam Wing and Fushigi Yuugi crossover all the characters were raised in New York City during the 20th century, that would be an AU. A good example of an AU is Lei's Earth, Moon, and Heaven.
Fusion: Where the characters from one (or more) series act out another series. Let's see - in my non-existant Gundam Wing / Fushigi Yuugi crossover, I think I'd have Heero replace Tamahome, Zechs replace Hotohori, Wufei replace Chiriko, Noin replace Nuriko, Duo replace Tasuki, Quatre could be Chichiri, Treize as Mitsukake, and Une as Mitsukake's cat.
Spamfic: Very short fic which illustrates a single point. Almost always silly.
Hmmm....I can't think of any more words I need to define. Can you?