Affiliate marketing is nothing else than a product or service promotion model that allows you to earn partners to sell someone else's products. The easiest way to explain this is with an example.
An online store selling electronics opens an affiliate program through which it wants to increase sales of its products. Anna, a blogger and photographer, wants to make money on her blog by selling cameras. She joins the affiliate program of the store mentioned above, downloads relevant links or banners from there, and places them on his blog.