Generally speaking, data mining techniques can be descriptive, predictive or both.
Most predictive techniques are also descriptive, whereas some techniques are purely descriptive.
What problem you're solving determines which technique you use, and the technique determines which products you buy. Predictive techniques, such as the classification technique, are probably the most commonly used. It's often used to isolate those most likely to respond to a direct mail marketing campaign, identify which possible transactions are most likely to be fraudulent, or forecast which products rolling off assembly lines are likely to be faulty. It also has interesting medical capabilities, such as the ability to identify people most likely to suffer from certain diseases. Predictive techniques are also used widely by long distance and cellular telephone companies to identify which customers are likely to leave.