RAIN uses behavior trees to control movement, animation, and general logic. Behavior trees are a physical way of representing logic, and there are many ways to accomplish the same task with them.
Tree structures generally involve a parent node with one or more child nodes. When a child node has no children it is called a leaf. When a child node has children of its own it is called a branch. The parent node that starts it all is called the root. This gives the appearance of a tree: The root has branches and the branches have more branches or leaves.
In the case of behavior trees: branches represent decisions, and leaves represent actions.