I think unsprung weight means anything which is not suspended by the suspension, e.g wheels tyres etc., and in general the less mass which is not under the control of the suspension the better, although of course the tyres themselves act as rudimentary suspension but offer limited damping etc! If you pick up a complete rear wheel assembly you will either think that a little extra weight on the sprocket won't make much difference, or you will think that the more weight you can save the better. They are heavy, especially with 18" rims and heavy tubes/tyres. I've never actually weighed tyres, but I always have the impression that Dunlops are lighter than most.