Categories vs. Tags: What’s the Difference?

Saw this question posted the other day on a forum for bloggers, and the asker seemed a bit embarrassed that she couldn’t quite grasp the concept:

What’s the difference between Categories and Tags?

Nothing to be embarrassed about; a number of new bloggers have difficulty getting it initially as well. I blame it all on being unfamiliar with new jargon, really, because it’s simple. Here’s essentially how I explained it to her, and it seemed to click for her instantly. Because again, simple:

Take a textbook. The Table of Contents chapters? Those are categories. The Index entries in the back of the book? Those are tags.

That’s it. Have fun!