The su command allows the user to switch to root.
grumpy$ wall Hello, World! wall: command not found grumpy$ su # wall Hello, World! Broadcast from root@grumpy (danamals): Hello, World! #
If the user does not have root on the host:
grumpy$ su Password: su: incorrect password grumpy$
No matter what password you enter, if you do not have root on the host, it will not allow you to become super user.