RAIN's behavior trees are usually comprised of two node types: decision and action nodes.
Decisions are containers that run other decisions and actions, while actions run tasks in the game world (animation, audio, mecanim trigger, etc). By making a tree of decisions and actions you can create some very complex behavior that is easy to drag around and modify visibly.
Decision nodes usually act as containers for other nodes (called “child” nodes) and are used to make decision about which actions to take and in what order. Examples of decision nodes are Selector, Sequencer, and Parallel.
Action nodes are responsible for “doing stuff”. These nodes initiate movement, drive animation, play audio, interact with game code, etc. Action nodes typically do not have child nodes. Examples of action nodes are Move, Animate, Play Audio, etc.
In addition to that there is one additional node called a Tree Binding which provides a way to run other behavior trees or to split your main tree into smaller reusable parts.