Search Engine Marketing (SEM) attempts to increase website visibility within search engines primarily through paid advertising.

Search engines primarily earn revenue from companies who purchase advertising space on Search Engine Result Pages (SERPs). This advertising space is many times at the top of the page and is the first item seen by a web searcher. Advertising cost is usually based on Pay Per Click (PPC) - every time a searcher clicks on an ad, the company that purchased the advertising space is charged anywhere from a few cents to a few dollars.

It is important to note that search engines do not concern themselves with a searcher's actions once they leave the search engine. Professional marketers will track the inbound traffic resulting from ad clicks and determine whether each click resulted in a conversion (filling out a form, purchasing a product, etc).