How Search Engines Work

The first basic truth you need to learn about SEO is that search engines are not humans. While this might be obvious for everybody, the differences between how humans and search engines view web pages aren't. Unlike humans, search engines are text-driven.
Search engines perform several activities in order to deliver search results – crawling, indexing, processing, calculating relevancy, and retrieving.

First, search engines crawl the Web to see what is there. there. This task is performed by e piece of software, called a crawler or a spider (or Googlebot, as is the case with Google). Spiders follow links from one page to another and index everything they find on their way.

After a page is crawled, the next step is to index its content. The indexed page is stored in a giant database, from where it can later be retrieved.

When a search request comes, the search engine processes it – i.e. it compares the search string in the search request with the indexed pages in the database.

The search engine starts calculating the relevancy of each of the pages in its index to the search string. There are various algorithms to calculate relevancy. Each of these algorithms has different relative weights for common factors like keyword density, links, or metatags.

The last step in search engines' activity is retrieving the results. Basically, it is nothing more than simply displaying them in the browser