The higher a website is "ranked" in search results, the more surfers will visit that site.
SEO can also target different kinds of search, including image search, local search, and industry-specific vertical search engines.
As a marketing strategy for increasing a site's relevancy, SEO considers how search algorithms work and what people search for. SEO efforts may involve a site's coding, presentation, and structure, as well as fixing problems that could prevent search engine indexing programs from fully spidering a site.
Other, more noticeable efforts may include adding unique content to a site, and making sure that the content is easily indexed by search engines and also appeals to human visitors.