What are Cremini Mushrooms?

I caught myself standing in front of the mushrooms at the supermarket last week asking out loud, "Now which one of you is the cremini mushrooms?" We call for cremini mushrooms in lots of recipes—I was shopping to make Orzo with Pecorino and Mushrooms—but I couldn't find them anywhere.

As I learned, cremini mushrooms are sometimes called "baby bella" mushrooms. If you can't find them, you can use white mushrooms instead in most cases. Sliced shiitake mushrooms are a good substitute, too, though usually pricier than the more common white mushrooms.