Search engine optimization is the process of improving the volume and quality of traget traffic to a web site from search engines via natural/organic search results for targeted keywords. Usually, the earlier a site is presented in the search results or the higher it "ranks", the more searchers will visit that site. SEO can also target different kinds of search, including image search, local search, and industry-specific vertical search engines. SEO can be approached in many ways, but the latest trend is Performance Based SEO.

As a seach engine marketing strategy for increasing a site's relevance, Search engine optimization considers how search algorithms work and what people search for specific terms. SEO efforts may involve a website's coding(HTML), presentation, images and structure, as well as fixing problems that could prevent search engine indexing programs from fully spidering a website. Another class of seo techniques, known as black hat SEO, use methods such as link farms and keyword stuffing that tend to harm search engine user experience. Search engines look for sites that employ these techniques and may remove them from their indices.

The initialism "SEO" can also refer to "search engine optimizers", terms adopted by an industry of consultants who carry out optimization projects on behalf of clients, and by employees who perform SEO services in-house. Search engine optimizers/experts may offer SEO as a stand-alone service or as a part of a broader marketing campaign. Because effective SEO may require changes to the HTML source code of a site, SEO tactics may be incorporated into web site development and design. The term "search engine friendly" may be used to describe web site designs, menus, content management systems, URLs, and shopping carts that are easy to optimize.