The word bark has two completely different meanings:

1. The rough outer covering of a tree's trunk and branches. Tree bark protects the living wood underneath from insects, disease, and weather, like a natural suit of armor. Some bark is smooth, like a birch tree's papery white covering, while other bark is deeply ridged and cracked, like an old oak's. Animals such as beavers gnaw through bark to reach the softer wood inside. People have used bark throughout history to make canoes, paper, medicine, and even cinnamon (which comes from the bark of a tropical tree).
2. The sharp, loud sound a dog makes. Dogs bark to alert their owners, express excitement, or warn strangers away. Different dogs have different barks: a small terrier might have a high, yappy bark, while a mastiff might have a deep, booming bark. People sometimes bark orders when they shout commands in a harsh, abrupt way, like a drill sergeant barking instructions to soldiers.