Search engine optimization (SEO) is the process of enhancing internal (HTML code, structure, content) and external (quantity and quality of referral resources) parameters focused on the improvement of site's positions delivered by a search engine on a concrete inquiry. SEO also includes different kinds of search, for instance image search, local search, etc.

SEO studies how search algorithms work as well as what people search for. It includes site's coding, its structure and presentation, different problems fixing. It performs also more unique features, such as adding content to a site, ensuring that it is easily indexed with the help of search engine robots, and of course making the site's look more eye-catching.

So it's worth starting with an introduction to the work of a search system. First and foremost, a search system consists of several components: spider, crawler, indexer, database and the most important part - results engine. The task of the last component is to define which pages meet a search demand and what order it should deliver results. The criteria, according to which a system takes final decision, are permanently improving, and this is the main problem of all optimizers. The SEO conception is build on operation algorithms of various systems.

On the whole, an optimizer's activity consists of correct work on external and internal parameters, influencing search results. They single out 3 SEO-activities classes: registration in catalogues and ratings, links exchange and other activities, improving quantity of links on a page. They can vary from one search engine to another.

Professionals are engaged only in "white" optimization, which includes only legal methods for site's promotion. "Grey" optimization, acceptable in some cases, implies application of unwanted methods of optimization: usage of automated systems of links exchange or purchase of links from subject sites.

"Black" optimization is extremely unwanted, it doesn't exclude the removal of a site from a search engine data base. "Black" are considered the following methods: hidden text usage (background color, tiny size) and dorveys creation. Dorveys are automatically created pages with senseless set of key words. Dorveys are used as brokers between a search engine and a promoted site.

Seo-activity has many pitfalls: spam filters, which check pages for spam and make fair optimizer's work hard; permanently changing algorithms of a results engine work; tough penalties and a great risk using even "grey" methods of optimization. SEO is a highly dynamic activity, which demands from optimizers new instruments, methods and understanding of the mechanism.

The leading search engines, such as Google, Yahoo! and Microsoft, use crawlers to find pages according to their search results. Search engine crawlers may consider a number of various factors crawling a site. Not every single page is indexed by search engines. Some search engines work with a paid submission service that can guarantee crawling for a certain fee or fee per click. Such kind of programs guarantee inclusion in the database, but do not ensure particular ranking within search results.