We like to classify anything: A, B and C customers; low-, medium- and high-risk credit risk applicants, species, races. We want to know if an email is spam or not? This is a typical classification for eMails.
Classification assigns an item (»object«) to a set of predefined classes.
If there is no set of predefined classes, we talk of clustering.
Techniques: Decision trees, nearest neighbour, neural networks, link analysis.