LightDM is a display manager used by various Ubuntu spins. Here's how you can install it in Ubuntu 14.04.
1. Open a terminal window.
2. Type in the following commands then hit Enter after each.
sudo add-apt-repository ppa:lightdm-gtk-greeter-team/stable
sudo apt-get update
sudo apt-get install lightdm-gtk-greeter
3. Use this command to restart your system.
According to Linux G:
LightDM is a lightweight display manager used by default on Lubuntu, Xubuntu and Kubuntu, responsable for the login screen, the management of the X servers and facilitates remote logins via the XDMCP protocol.More at Linux G
Additional Ubuntu resources