What is the difference between GDM and LightDM? How can one switch between them. I mean if I have selected LightDM during installation process, will I be able switch to GDM?
GDM (GNOME Display Manager) are login managers.
They both provide graphical login capabilities (the GUI with the list of the system's users) as well as remote logins using the XDMCP protocol.
Ubuntu is supposed to officially replace GDM in favor of the lighter and more easily customizable
You can have both of them installed (or add even more, like
KDM), but only one will be used at any time.
The default can be selected by running:
sudo dpkg-reconfigure gdm (if gdm is already installed)
More information on LightDM can be found on the Ubuntu wiki.