With Zazzle, you upload your artwork and place it on the different products they (prints, cards, t-shirts, etc) have so customers can purchase them.
You send customers to your Zazzle gallery with your designs that you've uploaded and when they make a purchase, Zazzle prints the item on demand and ships it directly to the customer.
This page explains how much you can make on each sale:
Make Money on Zazzle