Pay per Click is a type of search marketing where advertisers pay a set amount every time their ad is clicked by a prospect (otherwise known as a click thru).
Some search engines, such as Overture, specialize in this type of advertising medium, although pay per click is not limited solely to pay-per-click engines. For instance, Looksmart, a directory, recently changed its business model to pay per click. Also, XML trusted feeds through Inktomi and Fast are sold at a per-click basis.