Search engine optimization (SEO) - as a subset of search engine marketing seeks to improve the number and quality of visitors to a web site from "natural" ("organic" or "algorithmic") search results. The quality of visitor traffic 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. In effect, SEO is marketing by appealing first to machine algorithms to increase search engine relevance and secondly to human visitors. The term SEO can also refer to "search engine optimizers", an industry of consultants who carry out optimization projects on behalf of clients.
Search engine optimization is available as a stand-alone service or as a part of a larger marketing campaign. Because 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.
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. Some industry commentators classify these methods, and the practitioners who utilize them, as either " SEO", or "black hat SEO". Other SEOs reject the black and white hat dichotomy as an over-simplification.
Origin: Early search engines
Webmasters and content providers began optimizing sites for search engines in the mid-1990s, as the first search engines were cataloging the early Web. Initially, all a webmaster needed to do was submit a site to the various engines which would run spiders, programs that "crawled" a page and stored the collected data in a database.
By 1996, SEO related email spam was commonplace. The earliest known use of the phrase "search engine optimization" was a spam posted on Usenet on July 26, 1997.
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.
At first, search engines were supplied with information about pages by the webmasters themselves. Early versions of search algorithms relied on webmaster-provided information such as the keyword meta tag, or index files in engines like ALIWEB. Meta-tags provided a guide to each page's content. But indexing pages based upon meta data was found to be less than reliable, mostly because webmasters abused meta tags by including keywords that had nothing to do with the content of their pages, to artificially increase page impressions for their Website and increase their Ad Revenue. Cost Per Impression was at the time the common means of monetizing content websites. Inaccurate, incomplete, and inconsistent meta data in meta tags caused pages to rank for irrelevant searches, and fail to rank for relevant searches.  Search engines responded by developing more complex ranking algorithms, taking into account additional factors including: