The three most common ways in which online advertising is purchased are CPM, CPC, and CPA.
CPM (Cost Per Mille) or CPT (Cost Per Thousand Impressions) is when advertisers pay for exposure of their message to a specific audience. "Per mille" means per thousand impressions, or loads of an advertisement. However, some impressions may not be counted, such as a reload or internal user action.
CPV (Cost Per Visitor) is when advertisers pay for the delivery of a Targeted Visitor to the advertisers website.
CPV (Cost Per View) is when advertisers pay for each unique user view of an advertisement or website (usually used with pop-ups, pop-unders and interstitial ads).
CPC (Cost Per Click) or PPC (Pay per click) is when advertisers pay each time a user clicks on their listing and is redirected to their website. They do not actually pay for the listing, but only when the listing is clicked on. This system allows advertising specialists to refine searches and gain information about their market. Under the Pay per click pricing system, advertisers pay for the right to be listed under a series of target rich words that direct relevant traffic to their website, and pay only when someone clicks on their listing which links directly to their website. CPC differs from CPV in that each click is paid for regardless of whether the user makes it to the target site.
Source: http://en.wikipedia.org/wiki/Compensation_methods