Unsprung is what is says, weight that is unsprung, ie anything not supported by the springs.
Includes:
Tyres
Wheels
Brakes
Front hubs
Half the dampers/springs
Half the wishbones
Ends of steering rack.
Rear axle, diff and propshaft - these have me thinking. If solid and bolted to the leaf springs, is this sprung or unsprung? veIt is bolted to the spring but not at the springy part. However, its weight is supported by the spring so I guess it is sprung, but this means that the rear wheels, as they are mounted to the axle, are sprung as well. However, as wheels and tyres are considered to be unsprung weight the axle must be as well. Sometimes my brain hurts.
With reagard to performance, lighter is almost always better. The lighter the wheel, for example, the quicker it can respond to changes in elevation, direction and, to a lesser extent, acceleration and retardation (because the location of the weight plays an important part in these later cases)