A login can:
The database user is the identity of the login when it is connected to a database. The database user can use the same name as the login, but that is not required.
Create Statement: Create a login
CREATE LOGIN loginName WITH
PASSWORD = 'pwd';
CREATE LOGIN loginName WITH
PASSWORD = 'pwd',
DEFAULT_DATABASE = DefaultDatabase;
EXEC sp_addlogin 'userName', 'pwd', 'defaultDatabaseName';
GO
ALTER LOGIN login_name WITH PASSWORD = 'pwd'
SELECT * from master.sys.sql_logins;