I'm trying to understand how clustering and load balancing is achieved at the database level on an enterprise system. I understand how clustering at the application server level works, but it's not so clear at the database level. I read an article which mentioned "data replication" and "database clustering" and how you have a choice between the two when attempting to increase reliability and availability at the database level, but I was looking for more information.

If anyone knows of any good articles or links, please advise.