A simple defenition of unsprung weight is everything which it not being held up by the cars springs.
Such as your Brakes, Wheels & tyres. Increasing the unspring weight has an adverse effect on your acceleration and handling.
Imagine if you will how hard it is to spin a wheel on an axle and then compare that to a merry go round. in a similar way larger rims will cause your engine to work harder when accelerating.
Also if you take a bicycle and put it's front wheel in the air and spin it and then try turning the handlebars you'll feel that it's harder to turn while the weel is spinning rather than when it is static. Appy this to heavier rims and you'll soon understand why bigger rims can cause handling problems.
This is a very rudementary explination but i think it somewhat explains what i'm trying to say. Hope this helps.