Pay per click, or PPC, is an advertising technique used on websites, especially search engines. Pay per click advertisements are usually text ads placed near search results; when a site visitor clicks on the advertisement, the advertiser is charged a small amount. Variants include pay for placement and pay for ranking. Pay per click is also sometimes known as Cost Per Click (CPC).

In this area, a publisher is a webmaster that displays advertisements, from the advertiser. In order to bring together advertisers and publishers, a number of advertising networks were formed. They are often search engines, who already had man advertisers as clients, who they could link up with other publishers. The advertising network collects the money from the publisher, and passes a portion onto the publisher. Google is an example of this. Often these combined search engines and advertising networks, are (in the context of PPC) referred to simply as PPC engines.

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 being developed.

Advertisers using these 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 bidding.

Some of the PPC Keyword search engines are: Google AdWords, Genieknows, Yahoo! Overture (previously GoTo), FindWhat, GoClick, Enhance Interactive, 7Search, Kanoodle, ePilot, Search123, SearchFeed, Espotting, Xuppa, ROIClicks, RevenuePilot, IdoFind

"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.

Some of the PPC Keyword search engines are: BizRate, NexTag, PriceGrabber, Pricescan, Pricewatch, PriceLeap, Shopping.com

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

Examples: NexTag, SideStep, TripAdvisor