It’s actually quite simple. When you promote products as an affiliate you will be given a unique affiliate link. When people click on this link, it is tracked by cookies – and the affiliate commission is paid to you when someone buys something from that website after they have clicked on YOUR link. It varies across different affiliate schemes but it is not uncommon for the cookies to last for 12 months. So someone can click on your affiliate link and then not buy anything initially – but if they return to the website 10 months later and actually buy something you would still be paid for the sale. Not all schemes do this however so it’s wise to check this out before you promote.