Ubuntu and Debian

There are several ways to make the system boot to a tty prompt without
starting X. The most direct way is to edit the file

 /etc/X11/default-display-manager

and comment out the "/usr/sbin/gdm" or equivalent "kdm" line . Substitute
"false" without quotes. You could also simply clobber it with

echo "false" | sudo tee /etc/X11/default-display-manager    

You can edit /etc/inittab file to set custom runlevel for your system.