A bark is the loud, sharp sound a dog makes. Seals also bark, and people sometimes bark with laughter or bark out commands.
Bark is both a noun and a verb: if your dog is known for his noisy bark, it means that he barks all the time. There is also the type of bark that covers the trunk of a tree, and yet another bark, an old-fashioned word for a masted ship. When you "bark up the wrong tree," you have completely misunderstood a situation, the way a squirrel-chasing dog does when she literally barks in the wrong direction.