Starting with 12.04, Ubuntu doesn't ship with any screensavers, just a black screen that appears when your system is idle. If you’d rather have screensavers, you can swap gnome-screensaver for XScreenSaver.

Screensavers were actually removed back in Ubuntu 11.10. Ubuntu uses gnome-screensaver and inherited the change from upstream GNOME. The GNOME developers think a black screen that puts your monitor into lower-power mode is optimal.

Installing XScreenSaver

Open a terminal and run the following command to uninstall gnome-screensaver:

sudo apt-get remove gnome-screensaver

Install XScreenSaver and some additional screensaver packages with the following command:

sudo apt-get install xscreensaver xscreensaver-data-extra xscreensaver-gl-extra

Configuring your screensaver

After installation, perform a search in the Dash for Screensaver. Launch the Screensaver utility and use it to configure XScreenSaver and select your screensaver settings.

The Screensaver utility will prompt you to stop the gnome-screensaver process and launch the xscreensaver background process when you start it.

By default, XScreenSaver will choose a random screensaver each time it starts. You can specify the screensavers it chooses from or enable "Only One Screensaver" mode to always use your favorite screensaver.

Adding to Startup

To actually use the screensavers, you'll want XScreenSaver to start in the background each time you log in. If it doesn’t start, it can’t notice your system is idle and launch screensavers.

Get started by launching the Startup Applications utility from the Dash. Add a startup program with the following command. The name and comment here can be anything you like:

xscreensaver -nosplash

Reverting Your Changes

Reverting these changes is simple. Just run these commands to uninstall XScreenSaver and reinstall gnome-screensaver:

sudo apt-get remove xscreensaver xscreensaver-data-extra xscreensaver-gl-extra

sudo apt-get install gnome-screensaver

Source and more about: How to Add Screensavers to Ubuntu 12.04.