What’s the difference between descriptivism and prescriptivism?

Descriptive rules try to describe how native speakers of a language use it, while prescriptive rules tell speakers how they should use language.Many people think that it is “better English” to say “To whom were you talking?” rather than “Who were you talking to?”. The underlying “rule”, “Don’t put a preposition at the end of the sentence” is a prescriptive rule. However, it is perfectly grammatical (i.e. possible) to put a preposition at the end of a sentence (such as in the example above). It is thus a descriptive rule to say that in English you can put a preposition at the end of a sentence.