As the name implies, power is sent to all four wheels all the time (whereas a part-time system drives all four wheels only sometimes).

Full-time 4WDs can be driven on all road surfaces.

Some full-time systems are "selectable", so they can be run in two-wheel drive when the driver so chooses (such as around town and on highways).

Some have a selectable mechanical centre differential lock so they then deliver torque in a fixed 50-50 split front/rear for better off-road grip.