Same as cost-per-click, it is an online advertising payment model in which payment is based only on the number of click-throughs that are generated.
There is a similar payment model called a "pay-per-click search engine" (a.k.a. PPCSE) in which search engine results are ranked according to how much an advertiser pays to be listed, but advertisers are only charged if and when a userclicks on their listing.
In other words, pay-per-click ads work like this: You bid for certain keywords that describe your site and the products or services you offer. When a user searches with these keywords, they are brought to your listing. Money is paid only when someone clicks on the listing.
In 1998, Overture pioneered the idea of selling contextual-based advertising. Since then it has grown into one of the leading marketplaces in the world with Google's version known as AdWords. PPC listings are usually marked as "Sponsored Links" and appear above or beside the search engines organic listings.
Those in the industry advocate pay-per-click as an ideal short term strategy because of the immediate results and complete control.