Collection contain the hibernate some basic type, custom types, components and referneces to other entities. It is an important work. The collection of object can be handled with "value" or its life cycle fully depends on the collection owner, or it might be refernce to antoher entity with its own life cycle. We can work with only the "link" between the two objects is considered to be a state held by the collection.
We can say that the contained type is reffered to as the collection element type. Collection element are mapped by <element> or <composite-element> or in the case of entity references, with <one-to-many> or <many-to-many>. The first two map element with value semantics, the next two are used to map entity association.