Ubuntu 12.04 does not ship with any screen savers, just a black screen that appears when your system is idle. If you would like to have a screensaver, please follow the steps below:
Uninstall a gnome-screensaver.
sudo apt-get purge gnome-screensaver
Install XScreenSaver.
sudo apt-get install xscreensaver xscreensaver-gl-extra xscreensaver-data-extra
Create a symbolic link for Ctrl+Alt+L is the default shortcut that locks your computer.
sudo ln -s /usr/bin/xscreensaver-command /usr/bin/gnome-screensaver-command
Finally, add the xscreensaver -nosplash to the Startup Applications.