Read this pair of sentences:

I wore my green new coat yesterday.
I wore my new green coat yesterday.

If you grew up speaking English, you know that I wore my green new coat yesterday is not a correct English sentence. English has a rule for adjectives that requires more inherent qualities (such as color) to be placed closer to the noun than less inherent qualities (such as age). Even though you may never have heard this rule stated, you internalized it before you started the first grade. This rule is descriptive because it states what speakers of English recognize as English.

Consider another pair:

Don't blame the mess on me.
Don't blame me for the mess.

Both sentences may sound correct to you. Some people, however, avoid using blame on and prefer the second sentence. Rules that express preferences of one alternative over another are called prescriptive. Prescriptive rules deal with questions of usage.

You can think of prescriptive rules as a kind of etiquette of language. The more formal the occasion for writing, the more you want to pay attention to the fine distinctions of prescriptive rules."

-- from "Writing in the World," Chapter 8, section 32, in The Brief Penguin Handbook, by Lester Faigley.