PPC is an internet marketing formula used to price online advertisements. In PPC programs the online advertisers will pay internet publishers the agreed upon PPC rate when an ad is clicked on, regardless if a sale is made or not.
With pay per click in search engine advertising, the advertiser would typically bid on a keyword so the PPC rate changes. On single website or network of content websites the site publisher would usually set a fixed pay per click rate.
Pay per click has an advantage over cost per impression in that it tells us something about how effective the advertising was. Clicks are a way to measure attention and interest. Inexpensive ads that few people click on will have a low cost per impression and a high pay per click.
If the main purpose of an ad is to generate a click, then pay per click is the preferred metric. Once a certain number of web impressions are achieved, the quality and placement of the advertisement will affect click through rates and the resulting pay per click.