Shonen-ai is a Japanese terminology literally translated into "shonen" = boy and "ai" which means love. It denotes male homosexuality, and, unlike its correspondent yaoi, shonen-ai is used in regards to depictions of the romantic side of a relationship between two males. "Shonen-ai," the term, is used in fanfiction to warn of male homosexuality.
Often, shonen-ai is associated with fluffy, cute, and cuddly relationships - but the term is not completely restricted to such descriptions. Occasionally, an author will turn out a darker portrayal of a relationship between two guys and use the term "shonen-ai" as a forewarning that romantic relations between the two should be expected, but there will be very little (if at all) explicit sexual interactions. Unless specifically stated otherwise.
Shonen-ai also corresponds with the term "slash" which has less of an emphasis on whether or not the relationship between the characters involved is romantic or entirely sexual. Shonen-ai's counterpart is shoujo-ai - "girl"+"love". And the corresponding term for shoujo-ai that's used in regards to explicit sexual interactions between two females is yuri. Shoujo-ai and yuri can also be called femmeslash - or, sometimes, just slash.
"Bakura-kun gave Ryou-kun a Christmas kiss and chocolates and roses and presents and presents and even more presents - how typical shonen-ai-ish can they get!"
"Adrian and Lawrence are so cute as a shonen-ai couple and even if Adrian did killed Lawrence...he died in the end for Lawrence..."
Shonen-ai means any type of manga or anime that has a boy-boy love story. Basically a gay love story for girls who like guys who like guys. Shonen-ai is a form of shoujo. Not to be confused with Yaoi which is gay hentai.
Gravitation is shonen-ai bacause it is a love story between two guys.