A peek at Google's search quality team
By searchguy on Jun 03, 2007
The signals are not just the data from the page, but also metadata like the frequency of changes for a page and personalization information like previous searches you've run (if you're logged in, of course!)
The interesting thing is the description of how they evaluate queries. Each query is run against a number of classifiers that decide what kind of search it is and therefore what kind of pages to return for the search. This is probably one of those instances where having the logs of billions upon billions of searches and the computing power to analyze them gives Google a distinct advantage.
Also, keep in mind that all of this computation is done in far less than a second!