Affiliate marketing is the use of third parties to promote a product or service and encourage customers that might otherwise be unobtainable.
An example would be university listings on popular course search website Hot Courses. In addition to their own internal marketing activities, universities will post their courses on the Hot Courses website to drive more customers to their site. The university and Hot Courses become affiliated partners.
The affiliate is most often paid a fee (usually a percentage of the sale of a product or service), encouraging the affiliate to send as many customers to their partners as possible.