The fruit of the walnut tree, which grows in temperate zones throughout the world. The two most popular varieties of walnut are the English (also called Persian) walnut and the Black walnut. A close relative is the Butternut, also referred to as white walnut. English walnuts are the most widely available and come in many varieties -- some with moderately thick shells, others with shells so thin a tiny bird can crack them open. They're available year-round and come in three main sizes: large, medium and babies. When buying walnuts in the shell, choose those free of cracks or holes. Shelled walnuts should be plump, meaty and crisp; shriveled nutmeats are past their prime. Walnuts in the shell can be stored in a cool, dry place up to 3 months. Shelled nutmeats should be refrigerated, tightly covered, up to 6 months. They can be frozen up to a year. Walnuts are delicious in a variety of sweet and savory dishes and baked goods. They're also used to make a fragrant, flavorful oil.