In the US, paper weight is measured in lbs. and is determined by the weight of 500 sheets of the parent size of the stock. As an example, 20 lb. copy paper's parent size is typically 22x17" so 500 sheets of 20 lb. 22x17 weighs 20 lbs. Paper is also differentiated by the use of the words text or cover in the name of the paper; text being a lighter weight stock and cover being heavier. This method can be confusing as stocks will show as having the same weight even though one is heavier.
An easier way to compare paper thickness is using the metric weight. This is designated by the gsm (grams per square meter) which is the weight, in grams, of a 1x1 m sheet of a paper stock. This is a much clearer way to determine if a paper is thicker than another and is included on all paper types available through Print Services.
As an example, 80 lb. text is 118 gsm whereas 80 lb. cover is 216 gsm