You may have eaten ground walnut in desserts or in savory dishes such as pesto (it is a common and less expensive substitute for pine nuts), but it is actually the seed of the Juglans nigra - a northern hardwood tree that grows to as much as 100 feet in height.
The black walnut is covered with a green pulp until autumn, when it falls to the ground and turns black. Black Walnut Hull is the shell, or coating of the nut inside. It has long been important in Russian folk medicine as well as that of India and Pakistan, where it is known as akhort. In Croatia we make a healing “drink” called Orahovica from these medicinal hulls. 
Dried Juglans nigra may be taken in a capsule or used to create a tincture or liquid extract; however the taste is quite bitter.