amateur: Like in sports, amateur porn features individuals who are not professional porn stars. It features ordinary individuals partaking in every day sexual acts. Girl/boy next door, wives/husbands, girlfriends/boyfriends. Sometimes real amateurs but quite often professional models just pretending.
big dick: This kind of porn involves at least one performer with a massive penis.
college: College porn is a genre that focus on students. It features young performers an a college lifestyle: dorms, frats, sororities...