An affiliate program is a form of advertising, on the web, that rewards the affiliates (self-selected advertisers) for driving traffic to the advertiser (CTR) or for subsequent transactions (CPA). An affiliate program uses a multi-level marketing concept where consumers (affiliates) attract additional consumers. Some affiliate programs are multi-tiered, thus increasing the income earning opportunities for the affiliates (rewarded for their traffic and the traffic of affiliates they recruit). Marketers have the option of developing their own in-house affiliate program (Amazon.com and bn.com are examples) or joining one or more of the large affiliate networks (Beefree, Linkshare or CommissionJunction). Affiliates have the potential to earn significant money, if done correctly. This book store is the author's example of an Amazon affiliate. Marketing vehicles may sign up for an affiliate program to use its excess advertising inventory (unsold banners).