A Mahout-based collaborative filtering engine takes users’ preferences for items (“tastes”) and returns estimated preferences for other items. For example, a site that sells books or CDs could easily use Mahout to figure out, from past purchase data, which CDs a customer might be interested in listening to.
Mahout provides a rich set of components from which you can construct a customized recommender system from a selection of algorithms. Mahout is designed to be enterprise-ready; it’s designed for performance, scalability and flexibility.
Recommender Documentation – Apache Mahout – Apache Software Foundation