GNS Theory
A theory about what people want out of RPG's, sometimes called their Creative agenda(CA). The letter stand for Gamism, Naratravism and Simulationism. Gamist play is about over coming challenges, Naratavist play is about addressing a theme (no I'm still not quite sure what that means) and Simulationist play is about being part of the world and or celebrating the source material. (Personally I think this last lumps too many things into one)
The theory states that games that support only one of these CAs are cohertent, and game that don't have a clear CA are incohertent.
Also players can like multiple CA's but can only act out one at a time.
The theory has sparked a lot of debate, and long involved conversations about what it means. Many people find it a bit muddling. (I count as one of these so the above may not be quite right)
Links
Classic Essays (Most of these don't seem to be online any more)