For example, if an advertiser sells red widgets, he/she would bid on the keyword "red widgets", hoping a user would type those words in the search bar, see their ad, click on it and buy. These ads are called "sponsored links" or "sponsored ads" and appear next to and sometimes above the natural or organic results on the page. The advertiser pays only when the user clicks on the ad.

While many companies exist in this space, Google AdWords and Yahoo! Search Marketing, which was formerly Overture, are the largest network operators as of 2006. In the spring of 2006, MSN started beta testing their own PPC service, MSN adCenter. Depending on the search engine, minimum prices per click start at US$0.01 (up to US$0.50). Very popular search terms can cost much more on popular engines. Abuse of the pay per click model can result in click fraud.

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 do 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 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 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 PPCs

"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 PPCs

"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 PPCs have expanded into the service space while other service engines operate in specific verticals.

Pay per call

Similar to pay per click, pay per call is a business model for ad listings in search engines and directories that allows publishers to charge local advertisers on a per-call basis for each lead (call) they generate. The term "pay per call" is sometimes confused with "click to call"[1]. Click-to-call, along with call tracking, is a technology that enables the “pay-per-call” business model.

Pay-per-call is not just restricted to local advertisers. Many of the pay-per-call search engines allows advertisers with a national presence to create ads with local telephone numbers.

References: Wikipedia
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