It is called a tree because it is a hierarchical, branching system of nodes with a common parent, known as the root. As you've surely learned from reading this book, by now, behavior trees, too, mimic the real thing they are named after—in this case, trees. If we were to visualize a behavior tree, it would look something like the following figure:
A basic tree structure
Of course, behavior trees can be made up of any number of nodes and children nodes. The nodes at the very end of the hierarchy are referred to as leaf nodes, just like a tree. Nodes can represent behaviors or tests. Unlike state machines, which rely on transition rules to traverse through it, a BT's flow is defined strictly by each node's order within the larger hierarchy. A BT begins evaluating from the top (based on the preceding visualization) of the tree, then continues through each child, which, in turn, runs through each of its children until a condition is met or the leaf node is...