Bark is the covering of the stems of woody plants, like trees. Bark protects the tree. Bark of different plants and trees can look very different, it can be rough or smooth and can have different colors. It is the outer layer of tree trunks.
Most of the bark grows from the vascular cambium. Some outer parts of the bark sometimes come from a cork cambium. In the bark are lenticels which allow gases to move in and out of those inner tissues which have living cells.
Bark of one kind or another has been on trees from the Carboniferous period. It is obvious on stems and branches, and on some kinds of storage organs.