Hi,
Ok, here's the breakdown:
1. Login = To only access SQL Server, you first must have a login to server
2. User = To access a database, a login must be mapped to database. At this point, you become a database user via your login
3. Schema = This provides a way of organizing database object ownership, which is mapped to a database user
4. Role = This provides a way of controlling what a database user can do within context of a database
Thanks,
Phillip Cox