State the difference between internal nodes and external nodes of a binary tree structure.

An internal node of a binary tree is any node that has at least one child, i.e. it is not a leaf - it may have one or two children and lies within the body of the tree. An external node (also called a leaf node) is a node that has no children at all (both its left and right child pointers are null) - it lies at the boundary/end of a branch of the tree.