In botanical terms, the walnut is the large wrinkled edible seed of a deciduous tree (Juglans regiaspecies), consisting of two halves contained within a hard shell which is enclosed in a green husk. However, in commercial terms walnuts are almost never traded enclosed in a husk, but usually only after the husks have been removed and after some period of natural drying. Walnuts can be traded as in-shell (approximately one third of international trade) or shelled (approximately two thirds of international trade).

After the shell has been removed, the kernels are inspected and sorted, and can then be traded in different forms, such as halves, quarters, or pieces. Walnut kernels can be eaten either raw or baked as a snack, but in Europe walnuts are much more often consumed as an ingredient in home cooking and in food production than as a snack. The popularity of walnuts has grown considerably in recent years due to various health benefits being highlighted in the media.

Walnuts are grown in many different countries, but the leading world producers are China and the United States of America. The walnuts are harvested in the autumn when the green outer husks are starting to dry and split. The Northern Hemisphere crops (California, India, China, France, Italy and Eastern Europe) are harvested between August and November, while Southern Hemisphere crops (Chile, Australia) are harvested in March/April. Producing countries grow many different local varieties, but one of the world leading varieties is still Chandler, originating in the USA.