A Tree $T$ is a Graph that:

Trees can be

- Ranked or Unranked
- Ordered or Unordered

A tree is *ranked* if you know in advance how many children a node has

- Binary Trees are trees with rank 2 (see Binary Search Trees)

A tree is unranked if you don't know in advance the number of children

A tree is *ordered* if the order of children matters

For example,