What is a Pay Per Click search engine?
A Pay Per Click (also known as Pay Per Ranking, Pay Per Placement or Pay Per Position) search engine enables you to list your site at the top of the search engine results.

You pay only when a searcher or clicker (I use these terms interchangeably) clicks on your listing and connects to your site. You don’t pay to list or by the number of impressions generated, you only pay for clicks or click throughs.

You list your website by selecting keywords that refer to your product or service. For each keyword you determine how much you are willing to spend. The higher you bid the higher you will appear in the search results.

PPC search engines usually combine paid listings with unpaid listings to ensure any given search produces sufficient results. Typically, unpaid listings (also known in the industry as ‘fodder’) are provided by another search engine such as Inktomi, Google or the DMOZ project.

Most of the PPC search engines list the bid price in the results pages, but I believe it is more effective to keep bids off the results page. Listing bid amounts detracts from the perceived quality of the results and can unnecessarily bias a searcher’s behaviour.