This is one of those question that still gets asked all the time.
By default the Ubuntu root password is set to nothing so you can not login to that account.
Sometimes this can be a headache.
To change the password, login as the user you made during setup.
type: sudo passwd
and type the new password and confirm the password
Now the root account is set to that password you can now log in.
Note:
if you want to ssh as root you need to do the following
nano /etc/ssh/sshd_config
change:
PermitRootLogin without-password
to:
# PermitRootLogin without-password
save
Then restart SSH:
service ssh restart