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It is commonly believed that Google's initial success in leapfrogging the incumbent search engines early in its history was due in large part to PageRank, a web page ranking algorithm that paid attention to how other sites linked to a page to determine how important that page truly was. Eventually Google would expose the approximate PageRank of a given site using the Google Toolbar, which lead to a whole new way of valuing webpages. Previously, traffic was the only metric that mattered, but now with PageRank it was possible to see "important" a given page was.

Google has never published the secret sauce behind how PageRank actually works, although it is believed that PageRank's overall importance in determining search engine rankings has dropped considerably in response to people attempting to game the algorithm. Regardless, it still holds a certain mystique, and if you're interested in learning a bit more about what is currently known about PageRank, Smashing Magazine has an article you'll want to read called Google PageRank: What Do We Know About It? Since most of this is conjecture, take it with a grain of salt, but it appears some good work has gone into compiling this information.