Affiliate marketing is some sort of business relationship established between a merchant and his affiliates. In affiliate marketing, an affiliate agrees to direct some traffic to a merchant's website. If that traffic is converted into some kind of action, like a visitor purchasing a product on the merchant's website or a visitor becoming a lead for the company, the affiliate who directed the traffic will be compensated. Compensation may take the form of either a percentage sales commission for the sales generated or a fixed fee predetermined upon the application of the affiliate on the merchant's affiliate program.

Exactly as the title says: how to make money online selling other people's stuff!