verified amateur: As many videos are branded "amateur", which can also mean "low budget professional" lots of sites offer a "certified amateurs" category with only "real" amateurs.
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.
college: College porn is a genre that focus on students. It features young performers an a college lifestyle: dorms, frats, sororities...