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