i. definitions
ii. appearances
iii. theory
iv. order matters!
yaoi and shounen-ai are both from the japanese school of thought while slash is born from american fangirls. yes this is important. of course, these are usually written by girls, since boys tend to know what actually goes on.
i. definitions:
shounen-ai (少年愛【しょうねんあい】). i'll go more into the details of the japanese thought in a moment, but just, for simplicity, think of shounen-ai as what comes before yaoi. it's the cute pre-relationship whatnots, (as "pre-slash" is before "slash"), where the boys are paired but nothing happens.
yaoi (ヤオイ) is the japanese acronym for 'yamanashi, ochinashi, imninashi' or 'no climax, no conclusion, no meaning'. yaoi is actually a genre of shoujo manga/anime (manga/anime for girls), which is why it's easier to find in such media made for girls. and it's usually associated with anime, manga and fantasy-settings.
slash is the third male/male pairing thought that's usually associated with amererican tv shows and movies.
ii. appearances:
the first give away is how a pairing is written. in slash pairings, the boys are separated with a slash, hence the name (i.e. sirius/remus) whereas yaoi/shounen-ai use either a plus sign or an ex -- the latter means, basically, that they're together as a pairing already, screwing etc (i.e.. sirius+remus or siriusxremus)
iii. theory:
there are many things that point to the theory, and they are very different, yaoi/shounen-ai and slash. these include appearances of the boys and so forth. the main one, however, is the thought process. slash tends to deal heavily with the idea of homosexuality not being socially acceptable whereas, in the other two it's either not an issue or just accepted as fact from the start that boys like each other.
while there may be slash traditions and yaoi ones within a single fandom, another way to tell the difference would be by looking at the pairing. the yaoi tradition tends to pair boys together so that they look good where the slashers have some other way (i would explain, but i follow the yaoi tradition). an example of this would be that sanosuke and kenshin are paired together in yaoi (sanoxken) but usually it's slashed as saitou/sano. kenshin and sano look better but the slashers don't care. which is why most fanfiction is saitou/sano (since kenshin was dubbed in america) but all the doujinshi is sanoxken.
iv. order matters!:
this furthers the discussion in ii yet it deserves its own spot. in slash, the writers pick a name to go first, it's usually one that's parallel through the fandom. it could be, for them a/b or b/a without a difference. yaoi and shounen-ai matter!! vocab: seme (せめ) and uke (うけ). seme is from the japanese verb 'to attack' and uke is from the verb 'to recieve'. i won't take the time to explain what that means. but pairings are always, always written as semexuke or seme+uke. so axb is not the same thing as bxa. it doesn't make sense to have shuuichixyuki or quatrextrowa. so don't do it.
having said that... enjoy reading ^^*