Bark is the outer protective covering of tree trunks. The form and structure of bark can differ greatly from tree to tree.  As a result, it is a useful characteristic for tree identification.  

Bark is made up of two layers - outer bark and inner bark.

The outer bark is made up of dead cells.  This layer is usually quite thick, but in certain trees (young birch, for example) it is very thin.  

The inner bark, known as the phloem is made up of a thin layer of living cells.  These cells have extremely thin walls allowing water and nutrients (in the form of sugar solutions) to pass easily throughout the tree.  Somewhat akin to human skin, old bark is shed, and new bark is formed from the inside.

Leaves
Roots