The heart of Google software is PageRank, a system for ranking web pages developed by their founders Larry Page and Sergey Brin at Stanford University. It provides the basis for all of their web search tools. PageRank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page's value. In essence, Google interprets a link from page A to page B as a vote, by page A, for page B. But, Google looks at more than the sheer volume of votes, or links a page receives; it also analyzes the page that casts the vote. Votes cast by pages that are themselves "important" weigh more heavily and help to make other pages "important." Important, high-quality sites receive a higher PageRank, which Google remembers each time it conducts a search. Of course, important pages mean nothing to you if they don't match your query. So, Google combines PageRank with sophisticated text-matching techniques to find pages that are both important and relevant to your search. Google goes far beyond the number of times a term appears on a page and examines all aspects of the page's content (and the content of the pages linking to it) to determine if it's a good match for your query. Google does not sell placement within the results themselves (i.e., no one can buy a higher PageRank).

Google also trys to check for linking farms. If they see a lot of site are commonly getting linked back to it from a paid linking service this will actually lower your ranking

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