Skid Steering, aka Differential Drive or simply tank style, is a system where each set of wheels, or tracks is independantly powered.
To drive forwards(or back), both left, and right set must be powered forward (or backward).
To steer left, the right side must be going faster than the left, and to be able to pull an on-the-spot spin, the left should be put into reverse. The opposite applies to make a right hand turn.

The advantages of these is that you can turn on the spot, that you have can have a little more complicated wheel systems (like Tank Tracks, and Tri Star Wheels. They also lend themselves fairly well to robot control.

The disadvantages are that to a human used to a steering wheel style system - they are a little non-intuitive. Also, motors are not often identical, even in the same batch, so you may get some drift you need to correct after travelling a little - that problem is solved an the Adder Subtractor Drive.