Is it a tree?
Let's focus on this structure:
![](/img/16/bst02.png)
This is a tree! Imagine an upside-down tree where the root (and trunk) is at the top, and the branches are growing downward!
![](/img/16/bst03.png)
Resources
In the context of Graph Theory (Discrete Math), the structure above is called a "rooted" tree. Read the Wikipedia entry on Tree, Rooted tree for more information.
If you are interested in a detailed theoretical (mathematical) description of a tree, visit Chapter 4, Section 2 of Discrete Mathematics: An Open Introduction, 3rd edition.