Yaoi refers almost exclusively to a particular type of
doujinshi,
fan-drawn
parodies of popular
anime and
manga. Yaoi involves, by definition,
male/male sexual encounters, usually between characters that, in their original series, would not (to say the least) be involved in such
liaisons.
Ever wanted to see Mr. Satan ream Trunks up the back door? Welcome to the wide world of yaoi.
A joke has been made that yaoi is a short form of "Yamete kudasai, oshiri ga itai yo", but in reality it's an acronym: yama-nashi (no climax), ochi-nashi (no conclusion) and imi-nashi (no meaning).
It is distinguished from shounen ai by the fact that shounen ai has a point, real relationships are involved, hearts are broken, you know the drill. Yaoi is all about the nookie.