What’s the difference between 4 wheel alignment and tracking?

Tracking on the front wheels is sometimes called ‘two wheel alignment’. This does not take into account the direction in which the rear wheels are pointing. Therefore, if you have the front wheels adjusted and set straight and if the rear wheels are out of alignment then the car may pull and/or your tyres may still wear abnormally.
Tracking uses gauges where the operator peers through a ‘scope’ or views a light/laser beam on a scale. This system does not allow for run out compensation (taking account for any errors in the wheel rim), so the reading result can only be approximate at best. 
Four Wheel Alignment measures a minimum of 12 angles and compares them to the alignment data specified by the vehicle manufacturer. Wheel rim run-out compensation is taken into account, which gives accurate and repeatable readings. With such accurate readings, Four Wheel Alignment allows toe adjustments of individual wheels which ensure the steering wheel is set straight. Further adjustments of camber, caster and other angles (where necessary) can ensure optimum performance and savings.
On modern cars, tracking alone is is not enough and a full 4 wheel alignment/geometry is reccommended to deliver complete alignment.

4 Wheel Laser Alignment/Tracking and Geometry