Q. I have just installed Ubuntu Linux. But, what is the default root password? I can only login as a normal user.

A. This is the mystery for most users - you didn't set a root password, so what is it?

The root user can do anything as this a/c has full access to system. So using root account for daily work can be dangerous.

To be frank root a/c is locked by default under Ubuntu Linux. Therefore, you cannot login as root or use su - command to become superuser.

To run all administrative command use sudo command. sudo allows a permitted user to execute a command as the superuser or another user. Ubuntu setup your default account (the one created during installation) to run all administrative commands.

For example create a new user called bar, you need to type sudo command as follows:
$ sudo adduser bar
Password:

When sudo asks for a password, it needs YOUR OWN Password; this means that a root password is not needed.

Here are few more examples.

Task: Start / stop / restart services stored in /etc/init.d/ directory

$ sudo /etc/init.d/ssh stop
$ sudo /etc/init.d/networking restart


Task: Avoid typing sudo each and every time

Note that this is not recommended until and unless you are an expert and aware of what you are typing:
$ sudo -i

Above command will start /bin/bash as a root shell so that you can enter a root user command w/o using sudo.

http://www.cyberciti.biz/faq/ubuntu-linux-root-password-default-password/