While green tomatoes tend to be fried, and tomatillos are usually used in sauces, they look pretty similar on the outside: small, green, and firm. They're both also pretty tart, so are they really just the same thing, called by different names?
The Difference Between Green Tomatoes and Tomatillos
While both are members of the nightshade family, green tomatoes are hard, unripe tomatoes that can come from any variety of tomato. Tomatillos are not tomatoes, but the fruit of a different plant, and they are covered with papery husks.
More About Green Tomatoes
Green tomatoes are basically hard, unripe tomatoes that haven't changed color yet, with the exception of a few varieties of tomatoes that stay green when ripe (like Green Zebras). They're usually sold in the fall — when cooler temperatures mean the tomatoes don't get the necessary heat to ripen further and change color — although you can pick green tomatoes off the vine at any time.
Green tomatoes are firm and astringent, although the tartness mellows out with cooking, so they're usually cooked and most often fried.
More About Tomatillos
Tomatillos are the fruit of a different plant, and are sometimes called Mexican green tomatoes or jamberries. They are also green, although they can ripen to yellow. Tomatillos are coated in a sticky residue and covered with a thin, papery husk on the outside. Cooking helps soften the thick skin and brings out more flavor.
Tomatillos have a tart, fruity, and slightly herbal flavor. They're most often used in salsas and sauces, although they can also be eaten raw (and have a more acidic taste when raw). Tomatillos can be found year-round, most often in Latin American grocery stores.
Although they may look similar on the outside, green tomatoes and tomatillos are actually fairly different in flavor and usages, so I wouldn't recommend substituting one for the other. Tomatillos also tend to be juicier and not as firm, so they are quite different in texture from green tomatoes. Stick to finding the right one for your recipe.