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.