Search engine optimization (SEO) is a method used to increase the amount of visitors or viewers to a web site from search engines that comes from what are called "natural" ("organic" or "algorithmic") search results. Natural/organic/algorithmic results are those ones that appear appropriate to the content and context of the site, that have not been contrived or are irrelevant to the material they are related to. Usually the higher up the list or the sooner the result makes its appearance on the results page – the more visitors will call upon it. SEO has a variety of different search targets that include image search, local search, and industry-specific or category vertical search engines.

As an Internet marketing strategy, SEO figures out the way a search engine works and what it is that people are searching for. The optimization of a website is concerned most of all with the work of words, through content editing and HTML coding so that relevance is increased regarding specific keywords and any obstacles to the indexing activities of search engines are removed.

The abbreviation "SEO" may also mean "search engine optimizers," the people, specialist consultants who perform optimization projects for employees, or clients who provide in-house SEO services. They may perform SEO as an independent service or include it in a package for a total advertising or marketing campaign. Effective SEO might necessitate change to the HTML source codes of sites, so SEO tactics could actually become incorporated into the web site design and development itself. "Search engine friendly" can describe those web site designs, menus, content management systems and shopping carts that are easily optimized.

Black hat SEO or Spamdexing, are the highwaymen of SEO. They employ dubious devices like link farms and keyword stuffing , which undermine and belittle the relevance of any search results returned and the user-experience of search engines. Sites discovered using these devices are actively removed from the search engine indices.

SEO as a marketing strategy

Eye tracking studies reveal that searchers typically scan SERPs from top to bottom and left to right (for left to right languages), as they look for a relevant result. A position on or close to the top left of the rankings therefore increases the potential number of searchers who will visit such a favourably placed site. However, search engine referrals alone do not guarantee increased sales. SEO may not necessarily be the most effective strategy for every website, and other more appropriate Internet marketing strategies may be more efficient at returning sales, depending on the goals of the site operator. A successful Internet marketing campaign can direct organic traffic to web pages, but it may also require paid advertising on search engines and other pages, the building of premium quality web pages to engage and persuade visitors, addressing technical issues that are possibly preventing search engines from crawls that should index those sites, establishing analytics programs to enable site owners to measure their successes, and improving a site's conversion rate.

SEO may well generate a return on investment but, as search engines do not get any payment for organic search traffic, with algorithmic change there may be no guarantee of continued referrals. Too much reliance on (uncertain, non-guaranteed) search engine directed traffic could result in major losses if the search engines stop sending visitors. Wise business practice would suggest that website operators liberate themselves from their dependence on search engine traffic. A top-ranked SEO blog Seomoz.orghas reported, "Search marketers, in a twist of irony, receive a very small share of their traffic from search engines." Rather, their predominant source of traffic comes from links from other websites.

Webmasters and search engines

In 1997, twenty years after the standard protocol had been invented; search engines observed the efforts webmasters were making to swell their ranks in search engines, and noticed too that some webmasters had resorted to manipulation of results by cramming pages with an excess of or a contrivance of irrelevant or redundant keywords. Early search engines, like Infoseek, made adjustments to their algorithms as an attempt at prevention of the webmasters’ ranking manipulation malpractice.

Such is the extent of the marketing value of targeted search results; a potentially conflicting relationship could develop between search engine optimizers and search engines themselves. In 2005, an annual conference, AIRWeb, Adversarial Information Retrieval on the Web, was created in response to the threat with discourse aimed at minimizing the damaging effects of aggressive, even hostile, web content providers.

SEO companies employing such frowned upon methods may have their client websites banned from SERPs. In 2005, the Wall Street Journal reported on Traffic Power, a company allegedly using high-risk techniques that failed in its disclosure of those risks to its clients. Wired magazine further revealed that the same company sued blogger and SEO Aaron Wall for posting comments about the ban. Matt Cutts, the enforcer of Google’s Webmaster Guidelines later confirmed that Google did in fact ban Traffic Power and some of its clients.

Some search engines too have extended a friendly hand to the SEO industry, and frequently appear as guests and sponsors at SEO conferences, seminars and chats. Since paid inclusion, some search engines now have a seemingly symbiotic relationship with SEO and show a distinct interest in the welfare and success of the optimization community. Major search engines offer guidelines and information to assist with site optimization. Google has provided a list of suggested practices as guidance to webmasters in Google guidelines and has a Sitemaps program to help webmasters learn if Google is having any difficulties indexing their website and gives data returns on Google traffic to the website. Yahoo! Site Explorer provides a means for webmasters to submit URLs, determine how many pages are in the Yahoo! index and view link information.