Generally speaking, AWD refers to a full-time four wheel drive system. The difference between "full time" and "4 wheel drive" or "4x4" that the all wheel drive drivetrain has some sort of differential or slipping action between the front and rear axles to prevent "winding" -- a condition where the difference in the rotational speed between the front and rear axles caused by turning or breaking contact with the ground on either axle causes twisting forces to build up in the drivetrain. If not released, these twisting forces can ultimately cause driveline component failure and to a lesser extent, weird popping noises as the winding forces are released when a tire starts to break contact with ground. 4 wheel drive vehicles without differential action must be driven in the two wheel drive mode (either with locking hubs or a two wheel drive transfer case) in order to prevent "winding" edit: while driving on pavement.
The classic example of a full-time all-wheel-drive vehicle that has no normal, automatic way to relieve winding when driven on pavement is the British Alvis Stalwart 6-wheel drive Munitions Carrier. It has no locking hubs and all six wheels are connected to the drivetrain. Drivers have to regularly drive up on a curb or otherwise get a wheel into the air in order to release the strain on the drivetrain while driving on paved streets.
