An active anti-theft systems must be turned on or activated by the driver to be on and working. They are usually turned on by clicking a button on a remote or key. An active device requires you to activate it every time; this includes things like locking your door, putting a steering wheel locking device in place, or arming a car alarm system.
Passive devices automatically arm themselves when the vehicle is turned off, the ignition key removed, or a door is shut. No additional action is required. Examples of this type of system is the PASS Lock system, automatic locking doors when the key fob has move a predetermined distance from the vehicle.