To make it easier for you to manage your servers and clients, Microsoft Exchange Server is a scalable system based on the concept of organizations and sites. An organization is the largest administrative unit in Microsoft Exchange Server, and consists of one or more sites. A site contains one or more Microsoft Exchange Server computers that share the same directory information and are connected by a high-bandwidth permanent network. A small organization may have only one or two sites, whereas a large organization may have many.
Within a site, directory replication is automatic. However, you must configure directory replication between two sites in order for directory information to be replicated between sites.