GSM vs pounds (paper weights)

Papers are described by their weight, thickness and surface characteristics.

The weight is Grams per Square Meter (gsm), which is literally "how many grams would a piece of this paper one meter square weigh?" or "pounds per ream".

Of course, very few papers are actually a meter square, but GSM is a very useful way to compare the 'absolute' weight of various sheets of different sizes.

Pounds per ream is rather more troublesome, as it's not at all intuitive.
A 65# cover sheet is heavier than an 70# text sheet! And that's barely scratching the surface.
Different types of paper have different standard sheet sizes, so cover paper (ie thicker card-stock) is weighed as 500 sheets of 23x35, whereas bond paper is 500 sheets of 11x17, and 'text' paper is 500 sheets of 17x22

see http://www.limitedpapers.com/sizeweightchart.html