A polemic is something that stirs up controversy by having a negative opinion, usually aimed at a particular group. A piece of writing can be a polemic, as long as it gets someone's goat.
Polemic comes from the Greek polemikos meaning "warlike, belligerent." It's like challenging someone to a duel of ideas. These days a polemic is usually a piece of writing, such as if the Grinch published a powerful polemic against Christmas. It's like a debate, and philosophers from Nietzsche to Voltaire are known for theirs. The British philosopher John Stuart Mill had this to say about it: "The worst offense that can be committed by a polemic is to stigmatize those who hold a contrary opinion as bad and immoral men."