xfree - gdm - gtk+ - gnome

[tom <tom dbservice com>]

I dont know what is when called

if I execute 'startx', the xserver is started
but whats happening if I start gdm ?
gdm is a 'display manager', why we do need a display manager?
if I have two screens, where do I have to configure it, in xfree or in gdm?
and whats about the 'window manager'?
the same with the 'session' (gnome-smproxy)?

thanks
tom