Search engines generate revenue primarily through paid advertising. The great majority of this revenue comes from a pay-per-click (or cost-per-click) model, in which the advertisers pay only for users who click on their ads.
Since web searchers are free to use any of the many available search engines on the Web to find what they are seeking, the burden is on the search engines to develop a relevant, fast, and fresh search experience. For the most part, search engines accomplish this by being perceived as having the most relevant results and delivering them the fastest, as users will go to the search engine they think will get them the answers they want in the least amount of time. As a result, search engines invest a tremendous amount of time, energy, and capital in improving their relevance. This includes performing extensive studies of user responses to their search results, comparing their results against those of other search engines, conducting eye tracking studies (discussed later in this chapter), and conducting PR and marketing campaigns.
Because the search engines’ success depends so greatly on the relevance of their search results, manipulations of search engine rankings that result in non-relevant results (generally referred to as spam) are dealt with very seriously. Each major search engine employs a team of people who focus solely on finding and eliminating spam from their search results. This matters to SEO practitioners because they need to be careful that the tactics they employ will not be seen as spam by the search engines and carry the risk of resulting in penalties for the websites they work on.