college: College porn is a genre that focus on students. It features young performers an a college lifestyle: dorms, frats, sororities...
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.
babe: Usually softcore, this niche features girls that are considered extremely attractive. Beautiful and young female models.