The first letter refers to the location of the engine. The second letter refers to the drive wheels.

FF = front engine, front wheel drive

FR = front engine, rear wheel drive

RR = rear engine, rear wheel drive

MR = mid engine, rear wheel drive

Heard these for the first time in the Initial D anime.

There are also (rare nowadays and are mostly available in higher end / performance cars but used to be more frequent in late 80's and 90's):

FF 4WS FR 4WS

4WS is for «four wheel steering» (the emphasis is on «steering», not «driving») where – depending on the travelling speed – the rear pair of wheels turns either in the direction opposite to the front wheels (≤ 20 kmh) or in the same direction (≥ 20 kmh). The former makes the parking easier and reduces a turnaround circle. The latter makes the car look moving sideways when going around a bend or disappearing from its lane and emerging in the neighbouring lane during the lane change. 4WS also handles a bit differently on slippery roads around the bends.