Pay per click (PPC) is an advertising technique used on websites, advertising networks, and search engines. PPC is defined as the guaranteed placement of a small "ad" on the search results page for a specific keyword or keywords in return for a specified payment, but ONLY when a visitor clicks on that ad. The advertiser pays nothing to appear on the results page; they only pay the amount they have agreed to (or bid for) when someone actually clicks on their ad and is taken to their landing page; therefore, the term "pay per click".

PPC engines can be categorized in "Keyword", "Product", "Service" engines. However, a number of companies may fall in two or more categories. More models are continually evolving. Currently, pay per click programs does not generate any revenue solely from traffic for sites that display the ads. Revenue is generated only when a user clicks on the ad itself.

Keyword PPCs
Advertisers using this bid on "keywords", which can be words or phrases, and can include product model numbers. When a user searches for a particular word or phrase, the list of advertiser links appears in order of the amount bid. Keywords, also referred to as search terms, are the very heart of pay per click advertising. The terms are guarded as highly valued trade secrets by the advertisers and many firms offer software or services to help advertisers develop keyword strategies.

Product PPC’s
"Product" engines let advertisers provide "feeds" of their product databases and when users search for a product, the links to the different advertisers for that particular product appear, giving more prominence to advertisers who pay more, but letting the user sort by price to see the lowest priced product and then click on it to buy. These engines are also called Product comparison engines or Price comparison engines

Service PPC’s
"Service" engines let advertisers provide feeds of their service databases and when users search for a service offering links to advertisers for that particular service appear, giving prominence to advertisers who pay more, but letting users sort their results by price or other methods. Some Product PPC’s have expanded into the service space while other service engines operate in specific verticals