Classifying things is a natural human activity. We look outside the window and classify the weather into sunny, cloudy or raining. We watch football and classify the players into the defenders, the midfielders and the forwards, as well as into the two teams. We classify friends and colleagues into generations.
We do this because it makes it easier to think about them. We know what we might do on a sunny day, without going into the detail of just how sunny. We know what is expected of a defender in football. We know what we think of the music of Generation Y.
These are informal examples of what statisticians and marketers call segmentation. They are segments based upon our experience and we use them because they are useful.
When faced with new problems, particularly in marketing, data can help us define segments. This is often used to better target marketing, both to reduce costs and to improve effectiveness.