Yaoi is an acronym for yama nashi, ochi nashi, imi nashi meaning 'no climax, no punch line, no meaning', similar to PWP in fanfiction. There's no plot, just sex. Shounen-ai and BL are the fluffy stuff, usually focusing on romance and story. It's just that 'yaoi' became kind of a general term.
I like yaoi and shounen-ai, though. It's just hard finding good manga, imho.Most of the time the guys look too feminine, the stories are stereotypical and well-drawn porn is very very rare as well. Bara on the other hand is too manly for my tastes. A mix of both would be nice. Sadly it's close to nonexistent. I'm too picky, I guess. Slashing characters from existing manga and anime is the best alternative.