How to disable lightdm?
Note that the method described above doesn't work anymore since 15.04 and later, since Upstart was replaced by Systemd. As described here, one can now disable lightdm like this:
16.04 upward
systemctl disable lightdm.service
15.04, 15.10
systemctl disable lightdm
lightdm is started by Upstart, not SysV Init. So update-rc.d doesn't work.
Use
echo "manual" | sudo tee -a /etc/init/lightdm.override
If you want to disable lightdm to enable gdm or kdm or any other display manager, enter the following command in the terminal:
sudo dpkg-reconfigure lightdm
Then enter on ok
and then select from the list your desired display manager.
The same answer with screenshots is given here: Reset to Default Unity Login