When introduced, the Humvee military vehicle was a revolutionary combination of the best available technologies at the time. It combined the 4x4 four wheel drive with sophisticated electronics and thus became the most advanced and tough 4x4 available.

The civilian Hummer employed that technology to make a luxury four wheel drive which was capable of traversing roads and could occasionally be used off-road.

In addition to the capabilities of simple 4 wheel drives the Humvee had features that enhanced the off road performance of the vehicle. In the new Hummer technology not only the front and rear axel got the power from the engine but each wheel got 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]