Culled from a couple of websites:
Prescription is the laying down or prescribing of normative rules of the language. This is in contrast to the description of language, which simply describes how language is used in practice.
For example, a descriptive linguist (descriptivist) working in English will try to describe the usage, social and geographical distribution, and history of "ain't" and "h-dropping" neutrally, without judging them as good or bad, superior or inferior. A prescriptivist, on the other hand, will judge whether or not these forms meet some criterion of intelligence, rationality, appropriateness, aesthetics, or conformity to a standard dialect. Frequently this standard dialect is associated with the upper class (e.g., Received Pronunciation). When these forms do not conform — as is often the case for the "ain't" and "h-dropping" examples — the prescriptivist will condemn the forms, prescribing that they not be used.
i. Never split an infinitive
ii. Never end a sentence with a preposition
iii. Never use double negatives
i. Allophones of /p/ and /t/
ii. Prefix re-
iii. Pronouns and participle order
iv. Difference between 'paint' and 'clean'