The higher a page is visible/ ranked in a search engine has a direct correlation to the number of hits that web page receives. SEO uses a wide variety of mechanisms to augment a sites visbility in search results. SEO Professionals use a local search, google places, video search, news search and industry search for optimizing a site.
A SEO professional does intense survey of how the search engines operate for a specific keyword, what prospective users are searching and what results were exhibited for those sites. SEO also requires a website to be optimized and editing the site its keywords, meta tags, writing blogs etc.