X10 is a standard for carrying bi-directional data over home power lines, used in home automation protocols. In common use, modules plug into wall sockets and then have an appliance plugged into them. They receive (and return status) signals over the power lines and control the attached device. Control signals can come from a wired remote control, or from an outside device using either an RF to power line bridge device, or a computer interface device. Popular hardware was (is?) made by Radio Shack as well as a number of other vendors.
To implement computer control of X10 devices, you need some sort of gateway device between the X10 powerline network and a computer, and software to run it.
heyu uses the CM11A interface to send/receive signals. Using the CM11A, it can also log and act on all signals found on the line. This also allows the software to maintain a record of the state of all devices. It can also use the CM17A RF interface to transmit signals to an existing RF to X10 bridge. Both devices used serial connectivity.
Control software for CM17A
(prices, if listed, are from the linked vendor as of 08-2007)
TM751 Wireless (Bridge) Module) $13
KR19A Slim Pocket Remote $20
HR12A PalmPad 16 device remote $40
KR22A 4-device "credit card" controller $20
CM17A FireCracker serial-port wireless interface $40
FireCracker Kit - CM17A, HR12A, TM751, lamp module $40