In effect, SEO is marketing by appealing to machine algorithms to increase search engine relevance and ultimately web traffic. This is analogous to foot traffic in retail advertising. The term SEO can also refer to "search engine optimizers", an industry of consultants who carry out optimization projects on behalf of clients.

Search engines display different kinds of listings on a search engine results page (SERP), including paid advertising in the form of pay per click advertisements and paid inclusion listings, as well as unpaid organic search results and keywords specific listings, such as news stories, definitions, map locations, and images. SEO is concerned with improving the number and position of a site's listings in the organic search results.

SEO strategies vary widely, in accordance with the specific site. Broadly speaking, SEO may be geared towards increasing either, or both, the total number and quality of visitors from Search Engines. The quality of a visitor 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.

Search engine optimization is available as a stand-alone service or as a part of a larger marketing campaign. Because SEO of ten 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.

A range of strategies and techniques are employed in SEO, including changes to a site's code (referred to as "on page factors") and getting links from other sites (referred to as "off page factors"). These techniques include two broad categories: techniques that search engines recommend as part of good design, and those techniques that search engines do not approve of and attempt to minimize the effect of, referred to as spamdexing.

SEO, as a marketing strategy, can often generate a good return. However, as the search engines are not paid for the traffic they send from organic search, the algorithms used can and do change, and there are many problems that can cause Search Engine problems when crawling or ranking a site's pages, there are no guarantees of success, either in the short or long term. Due to this lack of guarantees and certainty, SEO is often compared to traditional Public Relations (PR), with PPC advertising closer to traditional advertising.