Crusty old SQL Server DBAs know all sorts of stored procedures for fixing up databases. A very useful one is sp_change_users_login, which you can use to map a database user to a server login after restoring a database from a backup.
You will need to do this when rebuilding a database server from scratch, because the backup does not store information about logins or any of the other server-level assets.
If you have to map the server login appuser to the SuperApp database user appuser, and you want to stay crusty, you can use
EXECUTE SuperApp.dbo.sp_change_users_login @Action = 'Auto_Fix', @UserNamePattern = N'appuser';
But you should prefer the new T-SQL language constructs, because the stored proc is going to be removed in a future version of SQL Server, and it is confisingly overloaded – it can change mapping or report on mapping depending on what value you pass to the @Action parameter.
The shiny new way top do the same is the ALTER USER statement:
USE DATABASE SuperApp; ALTER USER appuser WITH LOGIN = appuser;