In simplest form, X-10 is a system by which low-level control signals are 'piggy-backed' on the existing power lines of your home, in order to allow remote control of various lighting and appliances about the home.
X-10 refers both to the signals themselves (the 'protocol'), as well as the hardware of the transmitters and receivers that perform the switching, dimming, etc. Both manual and automatic control are possible.
Disclaimer: X-10 is extremely addictive.
Larry
www.fineelectricco.com