Search engine optimization (SEO) as a subset of search engine marketing helps to improve the number as well as quality of visitors to a web site from search results. The quality of visitor traffic can be measured by how often a visitor using a specific keyword leads to a desired conversion action, such as making a purchase or requesting further information. In effect, SEO is marketing by appealing first to machine algorithms to increase search engine significance and secondly to human visitors. The term SEO can also refer to "search engine optimizers", an industry of consultants who carry out optimization projects on behalf of clients.

Search engine optimization i.e SEO is available as a stand-alone service or as a part of a larger marketing campaign. Because SEO frequently requires making changes to the source code of a site, it is often most effective when incorporated into the initial development and design of a site, leading to the use of the term "Search Engine Friendly" to describe designs, menus, Content management systems and shopping carts that can be optimized easily and effectively.