Once again, Gojko Adzic comes with an excellent post : Improving performance and scalability with DDD.

Aggregates are often a bit underused in DDD because they’re difficult to grasp. They’re often seen as a solution to a technical problem. Gojko shows here how to understand them at a domain level. This post gives a clear vision of the role of Aggregates in DDD by placing it in the context of distributed environments.

You should read it !