Rashmi Sinha has a good overview of collaborative filtering: "... What is collaborative filtering? Technically, it's an algorithm for matching people with similar interests for the purpose of making recommendations. In non-technical terms, it's a system for helping people find relevant content. Unlike search, where you parse a query to and the most relevant content, with collaborative filtering you find some way of gauging an individual's interest in content, and then recommend what other similar users liked. ..." Link: Rashmi Sinha weblog.