I also see a weight associated with the art paper I buy.
For example: I have Bristol Board that is marked 100lbs, Pastel Paper marked 98lb, Sketch Paper marked 50lbs, etc.
What does this mean?
The weight is the weight of a ream (500 sheets) in the unit of uncut size, which varies with the stock type (see here, for example).
A more consistent measure of paper weight is the grams per sq metre value (gsm) which is what it says, the mass of 1 square metre of the paper, which is the area density of the paper.