Using a Dedicated Administrator Connection

SQL Server provides a special diagnostic connection for administrators when standard connections to the server are not possible. This diagnostic connection allows an administrator to access SQL Server to execute diagnostic queries and troubleshoot problems even when SQL Server is not responding to standard connection requests.

By default, the connection is only allowed from a client running on the server.

The DAC is available and supported through the sqlcmd command-prompt utility using a special administrator switch (-A). For more information about using sqlcmd, see Using sqlcmd with Scripting Variables. You can also connect prefixing admin: to the instance name in the format sqlcmd -Sadmin:<instance_name>. You can also initiate a DAC from a SQL Server Management Studio Query Editor by connecting to admin:<instance_name>.

For more details check: http://msdn.microsoft.com/en-us/library/ms189595.aspx