Behavioral design patterns, as the name suggests, are to do with behavior. Their purpose is to identify and implement common communication patterns between objects in an application. They define object interaction in such a way that the communication between objects is easy and coupling is still kept at a low level.
Behavioral design patterns describe how objects and classes interact with each other using messages. Contrary to creational and structural design patterns, the behavioral design patterns describe a flow or a process. This means that a developer should be really familiar with the actual process they are trying to implement. As with every other type of design pattern, behavioral design patterns exist in order to increase the testability, maintainability, and flexibility of the produced code.