Because of the ability to track results of online advertising at a more granular level than what is available through traditional advertising, varying ways have developed for the advertisers and publishers to do business. The three most common ways in which online advertising is purchased are CPA, CPC, and CPM.
CPA (Cost Per Action) advertising is performance based and is common in the affiliate marketing sector of the business. In this payment scheme, the publisher takes all the risk of running the ad, and the advertiser only pays for the media on the basis of the number of users who complete a transaction, such as a purchase or sign-up.
CPC (Cost Per Click) advertising is also performance based and is common in search marketing, where it is often known as Pay per click (PPC). In this scheme, an advertisement may be displayed (and assumedly viewed) many times, but the advertiser only pays based on the number of user clicks. This system provides an incentive for publishers to target ads correctly (often by keyword), as the payment depends upon the ad not only being seen, but the viewer responding and following the hyperlink.
CPM (Cost per Thousand) advertising is the most common basis in the business and is used for most display advertising and rich media. This scheme most closely resembles offline advertising, wherein the advertiser is paying for exposure of their message to a specific audience. CPM costs are priced per thousand, so that a $1 CPM, means that the advertiser pays $1 for every thousand impressions.