Search engine optimization (SEO), a subset of search engine marketing, is the process of improving the volume and quality of traffic to a web site from search engines via "natural" ("organic" or "algorithmic") searsc results. SEO can also target specialized searches such as image search, local search, and industry-specific vertical search engines.

Origin: Early search engines

Webmasters and content providers began optimizing sites for search engines in the mid-199os, as the first search engines were cataloging the early web.

Initially, all a webmaster needed to do was submit a page, or URL, to the various engines which would send a spider to "crawl" that page, extract links to other pages from it, and return information found on the page to be indexed. The process involves a search engine spider downloading a page and storing it on the search engine's own server, where a second program, known as an indexer, extracts various information about the page, such as the words it contains and where these are located, as well as any weight for specific words, as well as any and all links the page contains, which are then placed into a scheduler for crawling at a later date.

Current technology: Search engines consider many signals

To reduce the impact of link schemes, search engines have developed a wider range of undisclosed off-site factors they use in their algorithms. As a search engine may use hundreds of factors in ranking the listings on its SERPs, the factors themselves and the weight each carries can change continually, and algorithms can differ widely. The four leading search engines, Google, Yahoo, Microsoft and Ask.com, do not disclose the algorithms they use to rank pages. Some SEOs have carried out controlled experiments to gauge the effects of different approaches to search optimization, and share results through online forums and blogs. SEO practitioners may also study patents held by various search engines to gain insight into the algorithms.

Optimizing for traffic quality

In addition to seeking better rankings, search engine optimization is also concerned with traffic quality. Traffic quality is measured by how often a visitor using a specific keyword phrase leads to a desired convertion action, such as making a purchase, viewing or downloading a certain page, requesting further information, signing up for a newsletter, or taking some other specific action.

By improving the quality of a page's search listings, more searchers may select that page, and those searchers may be more likely to convert. Examples of SEO tactics to improve traffic quality include writing attention-grabbing titles, adding accurate meta descriptions, and choosing a domain and URL that improve the site's branding.

Relationship between SEO and search engines

By 1997 search engines recognized that some webmasters were making efforts to rank well in their search engines, and even manipulating the page rankings in search results. In some early search engines, such as Infoseek, ranking first was as easy as grabbing the source code of the top-ranked page, placing it on your website, and submitting a URL to instantly index and rank that page.

Due to the high value and targeting of search results, there is potential for an adversarial relationship between search engines and SEOs. In 2005, an annual conference named AirWeb was created to discuss bridging the gap and minimizing the sometimes damaging effects of aggressive web content providers.

Preventing search indexing

To avoid undesirable search listings, webmasters can instruct spiders not to crawl certain files or directories through the standard robots.txt file in the root directory of the domain. Additionally, a page can be explicitly excluded from a search engine's database by using a meta tag specific to robots. When a search engine visits a site, the robots.txt located in the root directory is the first file crawled. The robots.txt file is then parsed, and will instruct the robot as to which pages are not to be crawled. As a search engine crawler may keep a cached copy of this file, it may on occasion crawl pages a webmaster does not wish crawled.