This chart explains the weight differences very clearly:
How can I compare paper weights? - Paperandmore.com
The US measurement in pounds actually gives you no idea how heavy the paper is. A paper listed at 80# may be heavier than a paper listed at 110#. The most accurate measurement is in grams (and good luck finding that info!)
I just finished wedding invitations for my niece; I ordered the Ivory Antique Linen Cover shown here:
Antique Linen Cover - Marco's Paper
and the coordinating text weight paper to use inside the wedding programs.
Good luck in your search!