Granny Smith Apples are especially firm, tart apples. They are related to French crab apples, and are prevalent in North American baking.
Look for bruising on the surface, small (less than a dime) bruises are fine, but avoid apples with larger bruises. Apples should not have any tears in the skin, and should not smell like vinegar.
Granny Smith Apples are best known for baking well, due to their firm texture, they are one of the only apples that don’t get mushy when cooked. Their tart flavor also mellows when baked.