The xscreensaver program waits until the keyboard and mouse have been idle for a period, and then runs a graphics demo chosen at random. It turns off as soon as there is any mouse or keyboard activity.
This program can lock your terminal in order to prevent others from using it, though its default mode of operation is merely to display pretty pictures on your screen when it is not in use.
The benefit that this program has over the combination of the xlock (1) and xautolock (1) programs is the ease with which new graphics hacks can be installed. You don't need to recompile (or even re-run) this program to add a new display mode. Improvements to about a dozen modules, and a new module, nerverot.