Microsoft® SQL Server 2012 Unleashed

SQL Server Security: Users

Database users are principals scoped at the database level. Database users generally establish a link between logins (which are stored at the server level) and users (which are stored at the database level). Database users are required to use the database and are also required to access any object stored in the database.

Generally, the login name and database username are the same, but this is not a requirement. If desired, you could add a login named Chris and assign it to a user named Kayla. This type of naming convention would obviously cause some confusion and is not recommended, but SQL Server has the flexibility to allow you to do it. In addition, a user can be associated with a single person or a group of people. ...