Layout is a term for describing the placement of the main drivetrain components within a vehicle. The vehicle layout consists first of the location of the engine within the vehicle and then a description of the driving wheels (the wheels that receive the power from the engine).

FF = Front engine, front-wheel drive
FR = Front engine, rear-wheel drive
F4 = Front engine, 4-wheel drive
RR = Rear engine, rear-wheel drive
R4 = Rear engine, 4-wheel drive

Occasionally a vehicle will have a mid-mounted engine (meaning the engine is mounted behind the front axle and in front of the rear axle). The is noted with the addition of an M.

FMR = Front-mid engine, rear-wheel drive
RMR = Rear-mid engine, rear-wheel drive
RM4 = Rear-mid engine, 4-wheel drive