username - password for database

You didn't mention your SQLServer Version.

Use the following steps:
1) Open your SqlServer Management Studio > [YOUR DATABASE]> Security >Roles
2) Right click over "Database Roles", select "New Database Role".
3)Click on "Securables" and select other database roles, objects, etc to be assigned to that role. After that click "OK." The database role should now appear in the "Database Roles" listing under your database/security/roles.
4)Assign permissions to your database role. Only assign permissions to the database role that are absolutely necessary.

For further information please visit here
[UPDATED]
To Learn more about database role please visit the following links:

Database-Level Roles
Understanding SQL Server Roles
Creating User-Defined SQL Server Database Roles

Who are they?

You protect your database by using SQL Logins and roles and not publishing the password to those who don't need it. And don't ever use the SA account for access.

See these links my friend.

http://vyaskn.tripod.com/sql_server_security_best_practices.htm[^]

http://www.sqlsecurity.com/[^]

Regards,
Eduard