What exactly is SQL Server 2005 User Mapping?

In the new login dialog of the SQL Server 2005 Management Studio Express, what is the User Mapping actually doing? Am I restricting access to those databases that are checked? What if I check none?

From stackoverflow Lance Roberts

• It's mapping user rights to specific databases. If you don't check any, that user won't have rights to any database unless it is in a server role that allows rights to individual databases.

  From John Sheehan