These results are called Search Engine Results Page (SERP). Users normally tend to visit websites that are at the top of this list as they perceive those to be more relevant to the query. Some of these websites are at the top and others are not this is because of a powerful web
marketing technique called Search Engine Optimization (SEO).
SEO is a technique which helps search engines find and rank your site higher than the millions of other sites in response to a search query. SEO thus helps you get traffic from search engines.
Although the basic principle of operation of all search engines is the same, the minor differences between them lead to major changes in results relevancy. For different search engines different factors are important like for some on-page keyword factors have primary
importance while for others, links are very important and so on but main part is major search engines like different stuff and if you plan to conquer more than one of them, you need to optimize carefully.
Other popular sites that include search engines are – YouTube.com, Facebook.com, Amazon.com etc
Tags: What is SEO?