Search engines rely on complex ranking algorithms to decide the search result rank of any Web site. This process is dynamic and rankings are continuously updated.
Search engines also use "spiders" or "Web crawlers" to gather information about your site and bring it back to be analyzed by a powerful central engine.
Search Engine Optimization (SEO) refers to a diverse set of activities that you can perform to increase the number of desirable visitors who come to your Web site via search engines which is also referred to as organic medium.
In contrast to organic medium, a paid medium is one in which you are paying to bring visitors to your Web site.