Final answer:
Internal nodes in a tree have at least one child, while external nodes (leaves) have no children. The depth of a node signifies the number of edges from that node to the root, and the height is the depth of the deepest node. A subtree is a tree formed starting from any node and including all its descendants.
Step-by-step explanation:
Internal and External Nodes, and Other Tree Terminology
Internal nodes are nodes in a tree that have at least one child node. They are typically contrasted with external nodes, also known as leaves, which are nodes without children, residing at the bottom of the tree. The depth of a node is defined as the number of edges from the node to the tree's root node. On the other hand, the level of a node is the number of edges plus one, or equivalently the depth plus one, as the root node is considered level 1. The height of a tree is the number of edges in the longest path from the root to a leaf, which is also the max depth of the tree. Lastly, a subtree is a portion of a tree data structure that is itself a tree, consisting of a node and all its descendants.