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 ass: For big booty and massive buttocks lovers.
ass: Video genre that focus on the butts. It can includes anal, asslicking and rimming and even spanking, but also more generic movies that just focuses on asses, big or small.