Banners, sidebars and other predominantly-visual advertisements that appear on other websites. Display ads are facilitated by ad networks such as Google Adsense, Adnow etc.
Referrals from other websites with industry or product-focused content, such as reviews, comparisons, and testimonials. Successful affiliates have a loyal following or receive traffic from some of the above channels. They typically charge a set commission of referred sales, often determined according to the case.
Newsletters, abandoned cart notifications and remarketing all used and new emails to target past and potential customers.