Tomatillos are a close cousin to tomatoes, and grow inside a paper husk. They are ready to pick when they've grown enough to burst from the husk. They are firmer than a tomato, and green or green with a purple tinge, depending on the variety. They are used in a lot of traditional Mexican fare, and are what salsa verde is made of.
Want to learn more? Look up Tomatillo on Wikipedia.
STORE: In a plastic bag in the crisper drawer