Tomatillos look like green tomatoes, and though that's not technically what they are, they are often used in the same places you expect to find tomatoes - salsas, sauces for chicken and fish, etc. You'll most likely come across tomatillos in Mexican restaurants. They are tart in taste.
Small green fruits that, although they look like tomatoes, are actually members of the gooseberry family. If using fresh, remove the papery husks. Canned are a good substitute, but rinse well before using.