When one of your customers makes a purchase, you purchase the product from a third-party company (the drop shipper, usually a manufacturer or wholesaler) for a lower price. This process is as simple as forwarding the order from your customer, a process that can actually be completely automated. (Remember you don't have any risk here of buying inventory because the sale has already been made).
Affiliate marketing is a big and profitable industry that covers a wide spectrum of topics and fields. If you have got an interest in fitness, you can make money promoting fitness videos or courses e.g. videos to loss your weight. In fashion wear, you can earn commissions by recommending friends to clothing or t-shirt of companies which are offering affiliate programs.