Cricket - here's a link to a good picture of catNIP and some info. If you click on the picture there you will get a larger one. It also describes the shapes of the leaves and stems.
catNIP and here's a link to a good picture of catMINT. It looks like catmint leaves are more heartshaped and a little rounder, while catNIP is more elongated.
catMINT It would be funny if your plant wasn't either of these after all...
I'm just learing about the mint family - have catmint, chocolate mint, pineapple mint, and apple mint so far. Potted them all up except for the (older) catmint, so they wouldn't take over my universe!