BSS is a subtype of NTR. BSS is (as far as I understand it) where the protagonist has feelings (usually fairly strong feelings) for someone and then an antagonist comes along and seduces them, usually in a consensual manner in the media I've seen. I think it's very common in Korea (I first saw the term in relation to a Korean TV show).
It's distinct from ordinary netorare because there was never a relationship between the protagonist and his love interest (in fact, often times the love interest isn't even interested in the protagonist).