For some reasons, you may want to boot directly to the command line. You can easily do this by editing the /etc/default/grub file.
Edit your GRUB_CMDLINE_LINUX_DEFAULT= line, so it looks like this:
To apply this changes, you have to update your grub:
$ sudo update-grub
At reboot, you will get only a terminal. To turn on your Grafical Environment, do the following:
For Gnome: sudo service gdm start
For KDE: sudo service kdm start