X10 devices communicate over standard household wiring. The most common use is to control appliances and lights.
The computer communicates via a two-way power line interface (TW523), which is analagous to a modem. The TW523 plugs in to the wall and looks similar to a wall transformer/dc power supply for a portable radio. It connects via a modular phone cable and db25 adapter to the parallel port. With it the computer can send and receive commands such as ``turn on'', ``turn off'' or ``dim'' or ``select ID#''. Only receivers set to the selected ID will respond to the commands.
Receivers such as lamp and appliance modules plug into standard electrical outlets and the lamp or appliance plugs into the bottom of the module. Each module has a rotary switch on front which selects the module's ID.
The the tw523 costs about $18. The modules cost around $12-$15. A minimal setup consisting of a lamp module, tw523, modular cord and db25 adapter costs less than $40 at Radio Shack. Caveat: it's unwise to control things like garage doors, coffie pots or anything that could be a hazzard if turned on or left on when unattended.