Paid inclusion is a service offered by search engine as a search engine marketing product, where the advertiser is charged fees for inclusion of websites in the search index. Paid inclusion products are provided by most search engine companies.
The fee structure is both a filter against superfluous submissions and a revenue generator. Typically, the fee covers an annual subscription for one webpage, which will automatically be catalogued on a regular basis. A per-click fee may also apply. Each search engine is different. Some sites allow only paid inclusion, although these have had little success. More frequently, many search engines, like Yahoo!, mix paid inclusion (per-page and per-click fee) with results from web crawling. Others, like Google, do not let webmasters pay to be in their search engine listing (advertisements are shown separately and labeled as such).
Some detractors of paid inclusion allege that it causes searches to return results based more on the economic standing of the interests of a web site, and less on the relevancy of that site to end-users.
Often the line between pay per click advertising and paid inclusion is debatable. Some have lobbied for any paid listings to be labeled as an advertisement, while defenders insist they are not actually ads since the webmasters do not control the content of the listing, its ranking, or even whether it is shown to any users.
Paid inclusion is a search engine marketing method in itself, but also a tool of search engine optimization, since experts and firms can test out different approaches to improving ranking, and see the results often within a couple of days, instead of waiting weeks or months. Knowledge gained this way can be used to optimize other web pages, without paying the search engine company.