An all-wheel drive vehicle ( AWD vehicle) is one with a powertrain capable of providing power to all its wheels, whether full-time or on-demand.
AWD practically always refers to a four wheel drive system that is primarily intended for use on paved roads. These systems operate automatically with no intervention from the driver.
Some AWD systems include some level of control that optimizes function for road conditions, but no AWD system requires that the driver engages the control to enable four wheel drive.
All AWD systems have some form of mechanical, electronic, or electromechanical center differential that allow for speed differences between front and rear axles and use on dry pavement.
AWD systems are either full time (4 wheels driven) or part-time automatically engaging systems that send little or no power to one axle unless the other axle slips or the computer deems it necessary to transfer power.
Most performance AWD systems are full time systems. Some systems, like the Audi TT’s and Volvo AWD systems are part time but automatically engaging, sending a variable amount of torque to the second axle (generally the rear, though Porsche has a backwards setup on newer AWD 911 models that employ Porsche Traction Management)