Prescriptive - descriptive

In the study of linguistics, and in other academic areas, a firm distinction is drawn between prescribing something and describing it.

• To prescribe something is to lay down the rules, or to say what someone should do. This has always been a concern of schoolteachers.
• Linguists, or scientists of language, reject this approach. They see the aim of their subject as to describe language - recording the way in which it works and how it is used, rather than how it should be used. In this they follow the approach of other scientists who study human behaviour, like psychologists and anthropologists, as well as those who study other forms of animal behaviour.

Since the early part of the twentieth century, orthodox linguistics has concentrated entirely on the descriptive approach. The prescriptive approach is regarded as unscientific.

You may also want to clarify the difference between Prescribe and proscribe.