The Humvee military vehicle was a revolutionary combination of the best advanced technologies at the time. It combined the Four Wheel Drive with sophisticated electronics and thus becames the most advanced and tough 4WD available.

The civilian Hummer used that technology to make a luxury 4WD which was capable of traversing roads which suddenly acquired sand dunes, swamps, streams and other impediments

In addition to the simple 4 wheel drive the Humvee had features that can enhance the off road performance of the vehicle. In the new Hummer technology not only the front and rear axel gets the power from the engine but each wheel gets the power seperately from the engine thus all four wheels have the needed torque for the vehicle to give it's best performance

This can be easily understood by watching the following flash animation[Courtesy : HowStuff Works]