Google Algorithms

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Behavioral data analysis includes click-through rate, time spent on each site, pages visited and more. Google and other search engines monitor and analyze this data. On top of the tracking listed above, Google is likely to be monitoring users with a long history of specific searches.

[Admin]

If you have a Google account and use Google search on a regular basis, it adjusts search results based on the sites you visit the most, search queries you frequently use, blogs you track with Google Reader and other content you find/read using Google services. It uses this data to bring you partially customized results that are different from its regular results.
With this in mind, we can hypothesize that once Google has enough data from a pool of users, it can cross-compare and determine if a number of users with specific topics of interest share the same site-visiting patterns. If 1000 users interested in SEO read "SEO Chat" via Google Reader, then it's a good indicator that SEO Chat is an important website.
Based on collected behavior information, Google can adjust its search results. I would say that they would not go so far as to determine search results based on behavior, but they definitely use it to adjust and improve SERPs.
Microsoft Browse Rank is another example of how search engines use behavioral data. With the focus on behavioral data, now more than ever you need to build your site with your human visitors in mind. After all, that's truly the point of Google's algorithms: to return sites that human searchers will find relevant.