In the case of cost per mille/click, the publisher is not concerned about whether a visitor is a member of the audience that the advertiser tries to attract and is able to convert, because at this point the publisher has already earned his commission. This leaves the greater, and, in case of cost per mille, the full risk and loss (if the visitor cannot be converted) to the advertiser.
A two-tier affiliate program allows affiliates to not only earn commissions on their own sales, but to also get a percentage of the commissions (usually much smaller) earned by people they've recruited into the affiliate program (either directly because they knew them or indirectly – meaning someone signed up to be an affiliate by using the first affiliate's link).
Affiliate marketing is basically performance-based marketing, whereby affiliates/partners promote a merchant’s product/service and get remunerated for every sale, visit, or subscription sent to the merchant. The most frequently used payment arrangements include: pay-per-sale, pay-per-lead, and pay-per-click compensations. Affiliate marketing is one of the most powerful and effective customer acquisition tools available to an online merchant today. You decide what commission to pay, and pay only when results (sales, leads and/or clicks) are obvious.