Wolfram Research
Wolfram Math World: Tree
The mathematical description of a tree, with examples as a graph and as a data structure.
Other
Free Codecamp: How to Think in Graphs
In this article, we're going to demonstrate various applications of graph theory. These applications will contain detailed illustrations. As the author states, if you've been looking for a "graph theory and everything about it tutorial...
Wolfram Research
Wolfram Math World: Tree Leaf
Here is a description of tree leafs, with a table of data.
Wolfram Research
Wolfram Math World: Tree Spread
This site offers a very brief description of tree spread.