gay: HOMOSEXUAL. Refers to both homosexual men and homosexual women (also called LESBIANS).
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".
college: College porn is a genre that focus on students. It features young performers an a college lifestyle: dorms, frats, sororities...