Search engine optimization, or SEO, is a subset of search engine marketing, and deals with improving the number and quality of visitors to a web site from natural listings. Also SEO is an abbreviation for "Search Engine Optimizer" an industry of consultants who carry out optimization projects on behalf of clients. SEO is concerned with improving the number and position of a site's listing in the organic search results. Many SEO can provide useful services for website owners, from writing copy to giving advice on site architecture and helping to find relevant directories to which a site can be submitted.
 
In effect, SEO is marketing by appealing to machine algorithms to increase search engine relevance and ultimately web traffic. This is similar to foot traffic in retail advertising. 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 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.
A range of strategies and techniques are employed in SEO, including changes to a site's code and getting links from other sites .These techniques include two broad categories:
1) Techniques that search engines recommend as part of good design, and
2) Techniques that search engines do not approve of and attempt to minimize the effect of referred to as Spamdexing.
 
As SEO often 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. Search engine optimization is available as a stand-alone service or as a part of a larger marketing campaign.
 
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 searches can have algorithms that 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.