Bark is the protective outer covering that forms on the trunks and branches of woody trees. In botanical usage, it includes all tissues outside the vascular cambium, including inner bark and outer bark.
As a woody stem grows, its original epidermis is replaced by a periderm. The cork cambium produces cork cells whose walls contain suberin, a waxy material that helps reduce water loss and limits damage from pathogens and insects. Older outer layers commonly die and accumulate as the rough surface people recognize as bark.
Bark is not the same thing as wood, sapwood, or cambium. Wood is chiefly secondary xylem inside the vascular cambium; sapwood is the younger, water-conducting portion of that wood. The vascular cambium is a living growth layer, while bark lies outside it.
Bark also transports sugars through phloem and can store defensive chemicals. Cork from the cork oak, Quercus suber, is harvested for products without necessarily killing the tree. Some species shed bark in strips, flakes, or sheets as they grow.