both terms sound like a security system...the immobilizer..as ghunca said...is a system that operates via a micro chip in your car key.
immobilizer=something that disables mobility
if you put in another key in your car's key hole..the system will immediately disallow it and a sensor inside will send a signal to a brain / processor board (usually under ur dash) that a wrong key has been entered. in an instant...the immobilizer will kick in and will shut off fuel supply and will also disable the relay to the alternator...killing the ignition system.
