SQL Server 2012 introduces a new feature called Contained Database Authentication. The Contained Database Authentication feature allows the database to partially contain the data that was previously stored outside the database. In other words, once you enable the containment feature at the SQL Server instance level, you can create a database that can store user information/credentials inside the database itself (rather than having a login at instance level, which gets stored in the master database) and hence you don't need to create logins on the target instance (and do mapping of SID) again during database movement. Quite a relief...isn't it? For more information click here.