I just installed Ubuntu 13.10 on VMware. I am trying to login to root from my user account using su command but unable to login as Terminal is saying the following :
su: Authentication Failed
By default, the Root account password is locked in Ubuntu. This means that you cannot login as Root directly or use the su command to become the Root user. However, since the Root account physically exists it is still possible to run programs with root-level privileges. This is where sudo comes in - it allows authorized users (normally "Administrative" users; for further information please refer to AddUsersHowto) to run certain programs as Root without having to know the root password.
So if you want root access then you can use
Update :: To unlock root account
As @JosephR. suggested in comment, we can still become root or set root password using
then we can run