The Jeep robot's base
The Jeep robot is a front-wheel drive vehicle that has independent motors to power each wheel, as well as a third motor to handle steering. The motors are programmed so that when the steering motor changes its rotation angle, the left and right drive motors speed up and slow down appropriately to balance out the turn.
The Jeep robot can also avoid walls and other obstacles using the ultrasonic sensor.
For more about the book, read my Geekend post Geek Gifts 2011: The Unofficial LEGO Mindstorms NXT 2.0 Inventor's Guide.