A BehaviorTree chooses a behavior to be exhibited by an AI-controlled unit at any given moment. Behavior Trees are relatively simple to construct, but there is a lot of setting up to do to get one running. You also have to be familiar with the components that are available for constructing your Behavior Tree to do so effectively.
A Behavior Tree is extremely useful for defining NPC behavior that is more varied than simply moving toward an opponent (as shown in the previous recipe with AIMoveTo).