sudo is the command you want to do somthing as root type sudo then command then you the user password not the root password. That user has been assigned sudo root permission. or push alt-f2 and type gsku and the program you want to run as root. PLEASE READ
welcome to the sudo asylum. Ubuntu is set up this way so you do not destroy the system. And since you are asking this question you may want to learn how to use sudo. or try Debian or Slackware or fedora core or etc etc.
or enjoy the asylum. But if you read the Ubuntu Tutorials I linked to this reply you can have real root login.