Most copy paper is 20 pound. That is the basis weight of the paper. I will not go into the convoluted way this is calculated, but basically 4 reams of 20 pound copy paper weighs 20 pounds, hence the basis weight.
If you want to totally confuse yourself, see the link below.
http://www.paper-paper.com/weight.html
A ream is 500 sheets of copy paper, for all practical purposes.
Dan