If you are a regular user (that does not have elevated privileges) then you will not be able to run 'su - ' by itself to switch to another user's account. The root user, however, does have this capability. So if a regular user account wants to switch to another user account, they have to prepend the command with 'sudo' (provided that they have permissions in the sudoers file to do so).
To elaborate a little on Kenny's reply, if you were to run both of those commands as is, you would get a password prompt. Using just `su -` is going to prompt you for the root password, however `sudo su -` will prompt for your user password (assuming you have sudo perms). Both commands can be used to "su up" to the root user.
Thanks all of you . so one further question :)
Letting people login as root is that means that more people know the root password and have more than one way of logging in to the system. By making folks login as themselves, I have a little more control over what they can and cannot do - i.e. if I decide someone no longer deserves root, I can simply take them out of wheel - or lock their account - and I don't have to worry about changing root's password.