Shojo Ai is basically the japanese equivalent of shonen ai (boyxboy). So it's girlxgirl. It differs from yuri in that it's less hardcore. Like shonen ai is less hardcore than yaoi.

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed