The GNOME Display Manager (GDM) is something every GNOME user sees everyday - yet does not pay much attention to. It is the login screen which you see before you can get to your GNOME desktop. As far as I remember, Ubuntu has always had GDM. But now according to plans listed in the Ubuntu 11.04 "Natty Narwhal" blueprints, GDM might be replaced by LightDM.

LightDM is lightweight cross desktop display manager developed by Robert Ancell of Canonical. In the blueprint, several advantages of LightDM over GDM are listed:

• It is faster than GDM as it does not require the entire GNOME session to run.
• LightDM is cross-desktop. So Ubuntu and its various derivatives (Kubuntu, Xubuntu etc.) can have the same display manager.
• The LightDM codebase is around 5000 lines compared to more than 50,000 for GDM.
• It supports more usecases - first class support for XDMCP and multihead.

Although this has been accepted for Natty, the blueprint is marked as low priority at the moment. So, it is possible that LightDM may not make it to Natty.

[via Ubuntu Forecast]