According to About.com, four-wheel drive refers to a vehicle where all four wheels are driven by the drive train and two-wheel drive refers to a vehicle where either the back or front two wheels are driven. With four-wheel drive, the vehicle constantly provides power to all wheels.
There are several different types of four-wheel drive. For example, part-time four-wheel drive refers to engines that have the ability to shift to four-wheel drive on demand. Automatic four-wheel drive is a system where monitors are used to determine when four-wheel drive is needed and adjust the engine accordingly. Shift-on-the-fly four-wheel drive systems allow drivers to manually shift from two-wheel drive to four-wheel drive without stopping first.