A reverse harem includes three or more characters who potentially show romantic interest in a female protagonist. The sex, gender, or orientation of the harem members is irrelevant as long as they exclusively, or at least primarily, are vying for the affections of the same individual - who may or may not reciprocate towards one, several, or none of these romantic rivals.
Yaoi, known as Boys' Love or BL in Japan, is a genre mostly written by women, for women, that depicts homosexual relationships between men. Japan typically uses this single category for all forms of these relationships, sexual or not. In the West, the term Shounen-ai categorizes stories that focus on the emotional aspects of the relationships, while Yaoi categorizes more of the sexual aspects and explicit content. As Anime-Planet's audience is mostly based in the West, we use the Western definition for both Yaoi and Shounen-ai.