Theory is a complex mother-. It's that simple. Here are a few basic things I might mention soon, so...
D/F/K: Drama, Fortune, Karma. Three resolution styles. Drama uses description and talking to resolve things. It seems pretty rare to me, but Polaris is the first that comes to mind. Also free-form roleplaying. Fortune is some random element. Dice, cards, whatever. Karma is "the biggest guy wins the fight". Fortune is the most common that I know of, but it's not the only one. Games can also use some combination of the three.
G/N/S: Gamist, Narrativist, Simulationist. Three play styles or stances. Games can encourage or discourage any of the three. It might or might not be clear what each means, so ask if you're curious.
Lumpley Principle: Nobody definitively knows who came up with this. Some say it was Ron Edwards. Or Emily Care Boss. Or Vincent Baker (Lumpley himself). In any case, it basically says "Everything in any role-playing game system exists for the purpose of telling you who can say what and when.".
Narrative Control: Having control over the description of a scene. Traditionally monopolized by the GM-figure.
Scene Framing: The power of any person to set the beggining of a scene.
Stake Setting: Explicitly stating what's at stake in a given conflict.
There's probably a lot more I'm missing here, so feel free to ask. I might not know, but I might be able to point you in the right direction.