The common walnut trees are called Persian walnut as well. The official name is Juglans regia.
Walnuts are fruit, even though we call them nuts. The nuts themselves are called seeds.
Walnuts are used in salads and for sweets. They are popular in the US and have a distinct flavor. The nut itself is soft, not crunchy.

All the pictures are from Wikimedia Commons.

This is a common walnut tree in Desvojca, Kosovo  by, User: Durim shkodra.

Pamje_nga_DesivojcaThis shows a walnut in its husk, still on the tree by, Bohinger Friedrich.

360px-Juglans_regia_Echte_Walnußfrucht_1 Bohinger Frei drichWCThe rest of the pictures were taken by H. Zell.

Walnut tree.

Juglans_regia_001 tree H. Zell WCLeaf.

360px-Juglans_regia_014 H. Zell WC leafBark.

Juglans_regia_013 bark H. Zell WCMale bloom.

360px-Juglans_regia_003 H. Zell WCFemale bud.

320px-Juglans_regia_004 H. Zell WCYoung fruit.

360px-Juglans_regia_005 young fruit H. Zell WCWalnuts in their shells.

320px-Juglans_regia_009 H. Zell WCWalnut seed.

Juglans_regia_010 seed H. Zell WC