Affiliate marketing is a performance-based marketing, where an external partner is paid a commission for delivering a specific result to an advertiser. It can be a sale, a new newsletter subscriber, a download of an app and etc.
Affiliate Marketing is based on revenue sharing model. A company that has a product and wants to sell more can offer a commission to promoters through an affiliate program. While affiliate marketers, even though they don´t have any products themselves, can promote products that they like and earn an income from it!
To put it simply: Affiliate marketing is the process of earning commission by promoting products of other people and companies.