Yaoi presents a purely fantasized and highly romanticized take on m/m fiction...Traditionally, one lover will assume a dominant role in the relationship, becoming the “seme” (seh-meh), while the partner receiving the amourous ‘attentions’ is the “uke” (oo-keh). Yaoi is in a word: intense. The relationships might have complicated, angst-filled story-lines, but the action is graphic and designed to get the reader’s pulse pounding...The action doesn't necessarily have to be as gritty and realistic as possible---it's just done to entertain in a attractive and very erotically stirring way.
Keeping with the tradition of Japanese yaoi, English writers are trying to capture the gripping erotic action in book format. Visually descriptive and fast-paced, yaoi-themed novels strive to maintain the flavor of their counterparts across the Pacific.
--- excerpt from "Yaoi: A New Genre in Erotic Literature" by Anne Cain, used with permission. 
In Japan, the term 'yaoi' originally denoted inferior quality fan comics that were all porn with no plot, and the term is still considered derogatory by some today. The preferred Japanese term is "BL" (Boys' Love), which covers a variety of media, both amateur and professional.