Shoujo-ai means to focus on the relationship between two women. However it doesn't to design the sex explicit too much. Only the two women attract each other by cuddling, loving, kissing and emotional side of their relationship. In fact, it still is matured. In addition, the stories in particular tell the strong emotional bonds between schoolgirls as a mutual crush between an upperclassmen and underclassmen.
Shoujo means Girl(comics). Ai means Love.
GL mean "Girl Love"
In fact, I admit you that I read Shoujo-ai more than Shounen-ai.
Citrus and NTP=Netsuzo Trap continue more series