If you were to go online and start shopping, chances are you're getting plowed by many suggestions from online sites. However, these suggestions aren't random, but rather based on what you recently browsed and purchased. How did they determine what to recommend and what to ignore?
The system described above is called a recommendation system. The actual implementation, though, is through the use of a method called clustering. Clustering, in itself, is part of Cluster Analysis.
by Joseph Woolf