Within the general bracket of 'porn' - there are many specialized 'sub-categories' of adult entertainment. Think of 'porn' as a shopping mall, and the 'niches' as different kinds of shops within the mall. Not all surfers on the world wide web want to see blonde twenty-somethings, boning away with hard-bodied males. People have many varied sexual tastes and preferences, and use the Internet to explore them. Examples of some niches would be: big beautiful women (bbw), butts, breasts, spanking or interracial sex. There are many different niches around, and you can make good money promoting paysites in each of them.