Residual affiliate programs allow you to continue earning commissions after the initial sale is made. These are generally in place for products and services which have ongoing fees. Memberships which have monthly charges, and vitamins or supplements which ship out to the customer every 3 months are good examples.
If you acquired the customer as an affiliate and that customer continues to make regular payments for the products or services you referred them to, you will continue earning commissions from them. In most cases, residual affiliate programs last for the life of the customer. So if you sign up a new website hosting customer through a residual affiliate program, and that customer continues making their payments monthly for five years, you will continue earning monthly commissions for that entire five years as well.
Residual affiliate programs can be extremely profitable because new sales are combined with recurring commissions from previous sales.
