This will happen if you are using the separate timings for monitor sleep, system sleep, and hard drive spin down in the Energy Saver control panel. If the monitor is set to sleep before the system is set to sleep, the monitor is put into suspend mode. That is the video is turned off but one of the sync lines is still active to keep the monitor awake. The system itself is still functioning and moving the mouse or pressing a key will cause the video to return instantly. This is more of a screen saver feature than an energy saving feature although it does save some energy.
If the system continues idle for the amount of time set for system sleep, the monitor will be put into sleep mode. Actually, the whole system will be put to sleep. Selecting Sleep from the Special menu also puts the whole system to sleep. A key must be pressed to wake up the system at this point.