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.