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.
masturbation: Images of self stimulation particularly the genitals - see toys.
solo: Can be a video with just one performer, often masturbating or flashing. Solo can also be associated with another term, like "solo female" - a girls only scene, or "solo male".