Most search engines use text analysis tools to determine if a website matches a query.
The difference with Google is that it analyzes the relationships between popular websites and how they reference other websites to determine relevancy and ranking. Simply stated, the Google approach is based on the idea that you are likely to be important if other important people talk about you. That is, a website is likely to be relevant if other relevant websites reference it. Other benefits of Google are its simple, uncluttered interface, its speed, and the lack of any human tampering with search results. Unlike some search engines, Google does not let sites buy higher placement in the results that are returned.