+1 too much people make distinction between Shoujo ai and Yuri stating that if there are no sex, or lots of kisses then it's not yuri whereas initially IT IS called yuri >.>
It was always my understanding that Shoujo-Ai is SFW and Yuri is NSFW, much like Shounen-Ai and Yaoi. But like the definition of a tsundere, it's not the same as it was before.
I doubt there's really a definite dividing line between yuri and shoujo-ai, although I'd apply the latter to more subtle/innocent schoolgirl kind of stuff like Maria-sama and Miss Sunflower. Yuri, to me at least, covers everything with an clear statement of love (even just inside the character's head), and covers most of the NSFW stuff as well.
I'd personally call stuff with a strong influence of the real world, like Honey & Honey, Husky and Medley or Love My Life to be lesbian fiction rather than yuri. But I don't expect anyone else to follow my lead.