Paper thickness can be categorized in a number of ways. I tend to use the lb method, while other people may prefer ply or gsm. Let me break it down: there are a TON of different types of paper out there, but I usually stick with Crane's Lettra 100% Cotton Paper. I don't know why. I'm a creature of habit and it is what I've always used, and it works wonderfully. I'm secretly (or not-so secretly, if you know me) a 80 year old grandma. Change, no for heaven's sake!
Anyway, Crane comes in 110lb vs. 220lb. I've illustrated the difference in the photo below. 220lb is on the left, while 110lb is on the right. 220lb is thicker (hence the phrase 'double ply'), so it is traditionally more expensive- though in my mind, nothing quite beats a super-deep impression on 220lb paper.
Paper weight can also be described in GSM (grams per square meter), where traditionally with Crane's Lettra single ply/110lb paper is around 300gsm and double ply/220lb paper is around 600gsm.
Thus concludes today's lesson in paper weight. Oh, and if you are considering having something pressed and don't know which paper weight to choose? Shoot me an email and I'll snail mail you a sample of each so you can feel the difference in person.