Exchange Server 2010 brings a slew of features to your messaging and communication infrastructure such as: Archiving, Compliance, High Availability, Enhanced Messaging experiences from various entry points, and so on. High Availability’s one of the core themes of Exchange Server 2010. High Availability (HA) in Exchange Server 2010 combines replication techniques and Windows Clustering to deliver a highly and continuously available Exchange infrastructure. One of the key concepts in managing HA in Exchange Server 2010 is "Database Availability Group" (DAG) - a logical container for a set of Mailbox servers that provide isolation from database, server or network failures. Associated with DAGs is the concept of DAG Networks, that can be turned on or off for creating customized continuous replication and database seeding networks. Creating and configuring DAGs as well as DAG Networks, are core scenarios made easy and seamless in the EMC. The following screenshots show the DAG and DAG Network management experience in EMC.
Full Article: Exchange blog