“Search Engine Optimization is an active practice to optimize a web site by improving its internal and external aspects in order to increase the traffic that the site receive from search engines.”

How search engine operates?
1. Crawling the web (robots or spiders):  Estimates are that of the approximately 20 billion existing pages, search engine have crawled between 8 and 10 billon.
2. Indexing document: Once a page has been crawled, it’s contents can be indexed or stored in a giant database  of documents.
3. Processing queries: Match users’ queries by retrieving relevant documents from its index.
4. Ranking result: By using search engine’s algorithm to determine which is most relevant to the given query.

Source: SEOmoz.org