An affiliate program is a program set up by an ecommerce site to promote his offer on a large network of partner publisher websites called affiliates.
When partners (affiliates) sign-up to the affiliate program and display visual elements to promote the merchant offer, they are paid commissions according to the sales they bring to the merchant.
Affiliate programs may count many thousands of affiliates, if not more.
For a more detailed definition of affiliate program, see affiliate marketing.
An ad for an affiliate program:
Affiliate program principles: