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.
masturbation: Images of self stimulation particularly the genitals - see toys.