Prescriptivists are people who believe in and sometimes lay down rules for English usage. A descriptivist like me believes that anything commonly used by native speakers is correct by definition. To take a couple of examples:
1. It has been claimed by some that "between" should be used when there are exactly two cases, and "among" when there are more than two. But native speakers sometimes use between in the latter situation, and therefore in my view between must be considered good English.
2. Never end a sentence with a preposition. The (probably apochryphal) rebuttal attributed to Churchill was: that is the sort of pedantry up with which I will not put.