Walnut trees are deciduous trees, 10-40 meters tall, found across the north temperate zone and Argentina. They produce edible nuts, of which the primary type of nut found in stores are from Persian walnuts (which are easier to harvest because of their thin shells).
The nuts are rich in oil, and are eaten fresh and used in cooking, primarily in baked goods. Walnuts are also an excellent source of omega-3 fatty acids, and have been shown as helpful in lowering cholesterol. They need to be kept dry and refrigerated to store well; in warm conditions they become rancid in a few weeks, particularly after shelling.