Entities has associations to other objects, this may be an association to a single item (many-to-one) or an association to a collection (one-to-many, many-to-any).
At any rate, you are able to tell NHibernate to automatically traverse an entity's associations, and act according to the cascade option. For instance, adding an unsaved entity to a collection with save-update cascade will cause it to be saved along with its parent object, without any need for explicit instructions on our side.