Affiliate programs are setup by merchants to allow others to sell their offers. Amazon.com was one of the first affiliate programs I ever encountered online. Now millions of affiliates sell their books and other items worldwide. Clickbank.com is one of the best resources for affiliate programs. More details on products at clickbank
For every sale you make, you get a cut of the payment. This can be upto 75% with some products, usually digital products like ebooks or downloadable software, games, music etc.