From IAB Wiki
Fees advertisers pay Internet companies to list and/or link their company site or domain name to a specific search word or phrase (includes paid search revenues). Search categories include:
- Paid listings—text links appear at the top or side of search results for specific keywords. The more a marketer pays, the higher the position it gets. Marketers only pay when a user clicks on the text link.
- Contextual search—text links appear in an article based on the context of the content, instead of a user-submitted keyword. Payment only occurs when the link is clicked.
- Paid inclusion—guarantees that a marketer’s URL is indexed by a search engine. The listing is determined by the engine's search algorithms.
- Site optimization—modifies a site to make it easier for search engines to automatically index the site and hopefully result in better placement in results.