Somewhere we still have some of these - the last I saw they were tucked in a photo album of my mother's childhood. Green Stamps, also known as Victory Stamps, were given as credit for purchases at places like the grocery store. Those stamps could then be used as credits toward a reward at the S&H catalog.
Interestingly enough the company hasn't gone out of business - though they were purchased and updated for the advent of the internet. A full book of stamps - 1200 of them, will garner you a whopping $1.20 reward...hardly worth the funky taste in your mouth from licking 1200 stamps and affixing them to booklet pages.
Still S&H Green Stamps are an interesting historical artifact. Booklets and stamps are available in pretty good supply in auctions and places like ebay and could make for some interesting craft projects or ephemera decoration.