Pattern - oldest form of acrobatic competition. No rules on airplane appearance, depending on what years you are talking about, various rules on airplane size, engine displacement, and noise. Every plane in a given skill class flys the same routine in front of judges and is scored.

SA - Scale acrobatics. Same kind of flight rules as pattern (give or take a bunch), but the airplane can usually be any AMA legal size, but must be a roughly scale model of a real acrobatic airplane, like an Extra 300, Sukhoi, etc.

IMAC - International Miniture Acrobatics Club (I hope I got that right). The SIG (special interest group) of the AMA that is in charge of SA competition.

3D - A term (IMHO misnomer) for high-alpha post-stall maneuvers. In these moves, the wing isn't really lifting like in level flight, the plane is partly or entirely being held up by the engine, and controlled by prop blast on the control surfaces. The most obvious of these is a Torque Roll, where the plane is hovered stationary, nose pointed straight up, and rotates though a roll.

I'm sure there is a FAQ, but I don't know where

You can get more info on the kinds of competition on the AMA website or by checking out the forums dedicated to them, or even flipping though a print magazine.