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.
Sudo adds a log entry of the command(s) run (In /var/log/auth.log). If you mess up, you can always go back and see what commands were run. It is also nice for auditing.
You should never use sudo to start graphical applications as root. You should use gksudo to run such programs (eg. ALT+F2 gksudo gedit).
For users of Kubuntu, use kdesu instead of gksudo.
sudo adduser $user admin, where you replace $user with the name of the user.
sudo -i -u bobby --- to login as bobby
Create a launcher with the following command:
gksudo "gnome-open %u"
These instructions are to remove the prompt for a password when using the sudo command.
<your user name here> ALL=NOPASSWD: ALL