Te debo una disculpa, a veces no tengo mucho tiempo para contestar y no soy el mas indicado para dar explicacion sobre algunos temas. Asi que te dejo una definición y explicación mas acertada.
Cita: For each type of aggregate that needs global access, create a service that can
provide the illusion of an in-memory collection of all objects of that aggregate’s
root type. Set up access through a well-known global interface. Provide
methods to add and remove objects, which will encapsulate the actual insertion
or removal of data in the data store. Provide methods that select objects based
on criteria meaningful to domain experts. Return fully instantiated objects or
collections of objects whose attribute values meet the criteria, thereby
encapsulating the actual storage and query technology, or return proxies that
give the illusion of fully instantiated aggregates in a lazy way. Provide
repositories only for aggregate roots that actually need direct access. Keep
application logic focused on the model, delegating all object storage and access
to the repositories.
Si tienes mas dudas, puede ver este articulo:
http://code.tutsplus.com/tutorials/t...ern--net-35804
Espero eso ayude.