There are lots of changes in Ubuntu and one that you may have discovered is the lack of a screen saver. It is gone in 12.04. Screen savers were originally designed to prevent screen burn in, but that doesn't happen these days with LCD screens. Now they are simply fun displays that kick in when the computer is idle. You can easily add a screen saver to Ubuntu.
There are several ways to add a screen saver and the simplest is to use Ubuntu Software Centre. Run it from the Launcher on the left or the Dash and search for xscreensaver. Click the Install button and enter your administrator password.
Afterwards, click the Dash button and type 'screen' then click Screensaver. You'll see a warning that Gnome Screen Saver is running and it asks if you want to stop it. Click Yes. Then it says Xscreensaver Daemon isn't running and asks if you want to launch it. Say yes.
Unfortunately, you will see that it reports that very few or no screen savers are installed. Ubuntu Software Centre does a poor job of installing Xscreensaver.
Go to the Dash and run Synaptic Package Manager (if you don't have it, install it from Ubuntu Software Centre). Search for xscreensaver and select for installation everything that starts xscreensaver-blah blah. Click Apply and the selected files will download and install.
Run Screensaver again from the Dash and this time you should be able to select lots of screen savers. Set the mode to random screen saver or only one screen saver as you prefer. Select a screen saver to see it in the preview window and click the Settings button to configure its parameters (many have settings you can adjust that change how they work).
Unfortunately, this still isn't quite sufficient to get it working. It will operate until you shut down and then you have to manually start it the next time you boot up Ubuntu. The solution is to open the Dash and run Startup Applications. Click Add and enter
xscreensaver -nosplash into the command box. Set the name to Xscreensaver.
Remember that it reported that Gnome screen saver was active when starting Xscreensaver? We need to remove it. Go to Ubuntu Software Centre and search for 'gnome-screen'. Select Gnome screen saver and locker, and click Remove. That's it. The job is done and you now have a great collection of screen savers.
If you search the web you will discover that it is possible to do all of this from the Terminal command prompt. However, you shouldn't need to search the web for obscure commands every time you need to perform some simple task on your computer. You should be able to just point and click. Except for one command, we managed this with Xscreensaver.
It's a mystery why installing Xscreensaver from Ubuntu Software Centre doesn't work properly. You can actually skip that step and go straight to Synaptic Package Manager, which does the job properly. Well, it would if Ubuntu actually came with Synaptic Package Manager.