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.
hd: A BDSM abbreviation for humiliation and discipline. Also short for High Definition
babe: Usually softcore, this niche features girls that are considered extremely attractive. Beautiful and young female models.