Various search engines differ by how they determine which Web sites to list first in response to your search. This is called relevancy ranking. Relevancy ranking is critical because of the sheer volume of information on the Web. Most of us don’t have the time – or inclination – to sift through tons of Web sites before finding what we really want.
Most search engines use keyword frequency as the main way of judging if a Web page is relevant to your search. That means they simply count how often your keywords show up on the page. Some search engines consider both the frequency and the positioning of keywords. They assume that if your keywords are listed early in the document, or in the header, the Web page is more likely to be about your topic.
Google revolutionized relevancy rankings by considering the popularity of a Web page, that is how many other Web pages link to it. The idea being that if many people refer to this Web site it will also be relevant to you.