The xscreensaver program waits until the keyboard and mouse have been
idle for a period of time, and then runs a graphics demo chosen at
random. It turns off as soon as there is any mouse or keyboard
activity. It can also lock the screen immediately, after a longer idle
period, or on demand.

The xscreensaver package consists of two parts: the screensaver and the
"driver" or "daemon", which detects idleness and does locking, and the
many graphics demos that are launched by xscreensaver.

Any X program that can draw on the root window can be used with
xscreensaver, regardless of how that program is written, what language
it is written in, or what libraries it uses. The xscreensaver daemon
takes care of detecting when the user is idle, locking, and checking
passwords and all the other book-keeping. All the other programs need
to do is draw.

The benefit that xscreensaver has over the combination of the xlock and
xautolock programs is the ease with which new graphic hacks can be
installed. You do not need to recompile (or even re-run) the
xscreensaver program to add a new display mode, you just change a
config file.