screen saver 
noun
An animation or image which replaces an unchanging computer display after a period of inactivity.Password-protect your computer when the screen saver is active, so when you're away from your PC, your computer will be locked until you enter the password....
- Other times, the spyware is secretly hitched to another program that the user does want - often a free screen saver, game or peer-to-peer client.
- This utility is a wonderful screen saver which shows your favorite nature pictures collections at the computer's idle time.