Confused Logins and Users in SQL Server
A SQL Server login is used to authenticate the principal at the server level, and to provide server level permissions.

A database user maps to zero or one login and is used to provide a security context within the database.  Database permissions are assigned to database principals, including roles, not directly to logins.

User names are generally not specified in connection strings.  The exception is when a user is authenticated at the database level, and therefore has no login.