Here's a pretty good definition of unsprung weight.

"An important concept is "unsprung weight." This is weight that is not supported by the suspension of the car. This usually includes the the weight of the wheels and tires as well as a percentage of the weight of the suspension itself, including control arms, anti-roll bars, shocks, and struts. Reducing unsprung weight is the key to improving handling. The lower the unsprung weight, the less work the shocks and springs have to do to keep the tires in contact with the road over bumpy surfaces. An easy way to reduce unsprung weight and improve traction is to replace stock wheels and tires with special lightweight wheels. Note that as the wheel diameter or width increases, the weight of the overall wheel and tire package increases, thereby increasing unsprung weight."