Fictional homosexual pairings, usually somehow related to Japanese culture. A.k.a. drawings and stories of male characters giving each other head and fucking each other in the ass. Even though the content is adult, explicit, gay male sex, pretty much the only people who create or appreciate yaoi are straight female children. Don't ask me why.
13-year-old girl: OMG did u see that new yaoi story xXhakuri-chanXx rote about Draco Malfoy doing Harry up the ass and then cumming all over his face while Snape watched and jerked off?! KAWAII!! ;;^__^;;