Ad Hominem and Related Beasts:

The ad hominem fallacy has several components, including the direct (or abusive), circumstantial, and tu quoque varieties. The general public usually thinks of the direct version and sometimes confuses it with ad personam. The direct version is a mistake in formal debate and indeed becomes abusive in courtrooms, scholarly discussion, and other arenas. On the other hand, the circumstantial version of ad hominem is essential for calling out things like hypocrisy.