In the U.S. and Canada, paper weight is specified using an arcane system based on 500 sheets of a basis size (or "basic size") of a named category of paper: bond, text, cover, etc. Business supply stores will use the bond weight (most common: 20lb, occasionally 24, 28, 32 are also available). Professional printers will more commonly use the text (also called offset) weight to talk about the same paper.
table of the most common equivalent weights
(conversions are approximate, especially GSM (metric g/m2); not all equivalents are commonly manufactured; actual GSM varies between manufacturers)
| Bond | Text | Cover | GSM |
|---|---|---|---|
| 20 | 50 | - | 75 gsm |
| 24 | 60 | - | 90 gsm |
| 28 | 70 | - | 106 gsm |
| 32 | 80 | 45 | 119 gsm |
| 40 | 100 | 56 | 151 gsm |
| 47 | 120 | 65 | 177 gsm |
In my experience (in Canada) 80, 100, and 130 lb. cover are also common, but you wouldn't see it sold under text or bond weights.
(i graduated 2 months ago, i thought i was free of this ordeal)