The Database Availability Group (DAG), a high availability feature of Exchange Server 2010, provides database-level recovery from failures and data corruption.
The DAG is a group of up to 16 servers in which some or all of the mailbox databases are replicated to one or more servers within the DAG. Thus there can be up to 16 copies of a database in the DAG. A database is not associated with any server. The current active database and its copies use the same path on each server. All database names in a DAG have to be unique.
The DAG is for mailbox databases and not for public folder databases.