I've gotten this question a few times since mentioning it.
It's this ridiculously fun class that involves dance, aerobics, and great music.
Here's a You Tube clip that goes into what a Zumba class looks like. Get thee to one pronto.
It's really high energy and fun. I went through a whole bottle of water at my first class this morning. At the 1/2 hour point I was thinking "How am I ever gonna get through a whole hour of this???" But you know what? I did. One hour of jumping, grooving, shaking and tapping my fat away. It was a blast. I want to go twice a week now. :)