Edit /etc/default/grub with your favourite editor, e.g. nano:
sudo nano /etc/default/grub
Find this line:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
Change it to:
GRUB_CMDLINE_LINUX_DEFAULT="text"
Update GRUB:
sudo update-grub
This is an additional step for systemd releases, e.g. (L)Ubuntu 15.04, the steps above for grub are still necessary.
You need to tell systemd to not load the graphical login manager:
sudo systemctl enable multi-user.target --force
sudo systemctl set-default multi-user.target