Ubuntu is one of the few Linux distributions out there that will not enable the root account.If you want to do something with root permission on the console you have to type sudo before the command.
sudo" means superuser do. "sudo" will prompt for "Password:". Please specify user password
As you have noticed during the Ubuntu installation there was no question about the root password, as you might have been used to see during other Linux distribution installation process.Because of this your root accout is inactive.
If you want to enable root account (which is not recommended) enter the following command.
$sudo passwd root
This will prompt for a new root password and once you confirm it, you can start using the root account to login.
If you want to disable root account in ubuntu you need to lock the root account by using the following command
$sudo passwd -l root
If you want to work on a root console you'd better use the following command