In order to log in to a Linux system a user must have a user ID. This is assigned by the system administrator and usually matches the user's name or position (e.g. SmithB or Acctg). User IDs have a password associated with them that only the user themselves should know (not even the system administrator should know this password). Try thinking of the user ID/password combination as a key that lets a user into the system.

There is a special user account, called the Superuser account, that can be used by system administrators. This account is allowed to do anything on the system, but it is generally used to run system administration tasks. With the power that has been bestowed on the Superuser it is easy to understand why you need to be very careful about who gets the password to this account.