This is one of the most important features, as almost everything in the engine is based on entities having a variable number of behaviors.
While an applications is running, all of these are excuted in an order based on their categories RapidUpdate, Update and OnDraw, for (in that order) high-priority updates, standard-priority updates and rendering.
Behaviors, unless defined as a substantial part of the entity, can be started and stopped at any given time, the result usually applied immediately in the next frame, same for creation and disposal of entities, resulting in a rather solid and versatile codebase and promoting reusability both in engine-code and application-code.
Tutorials about Entites can be found on the tutorial page
All details and guidelines about the Entity Component System can be found in the wiki