wiring. Modules consist of controllers, receivers, and transmitters. Controllers can be wired (i.e., replacing standard wall switches), wireless (where the receiver is plugged into an AC outlet), or computer controlled (where a serial or USB port is connected to a AC outlet interface). Receivers can be plug-in modules, replacement wall switches, replacement AC outlets, direct-wired into the house wiring or device, etc. Transmitters can be any of the above plus RF, IR, and other crossover methods.
More advanced X-10 systems incorporate security systems, IR transmitters & receivers, and extensive computer automation to tie everything together. Anything you can imagine can be monitored or controlled via various combinations of the considerable available modules, like drapes, driveway sensors, mailbox sensors, garage door closers (several of us went back and forth years ago on what was the most efficient way to check if the garage door was open and if so close it when the system did its nightly bed check.) I've had mine maintain my pool pH, notify me via text msg if my basement flooded, automate my HVAC to turn the heat down after I go to bed and back up an hour before I wake, an away mode turns lights on and off thru the house to appear occupied when I'm away, my main computers speak to me when certain sensors are tripped while the others chime one tune for the front door and another for the back, the main automation computer runs programs whenever certain conditions are met with the automation system, scenes can be created for one button control(for instance a movie night scene shuts off certain lights, dims others, closes the drapes, engages the movie screen, starts the projector, etc.), my rain sensor disables the sprinkler system, and the list goes on and on and on.
Smart Home is a great resource to find out what the latest and greatest modules are, but I rarely buy from them (Too expensive). X10.Com is the cheapest online retailer of x-10 modules, but there are several other brands that are compatible (Sears, Radio Shack, HomeVision, Leviton, Insteon, etc). I use ActiveHome software cause its cheap and does most of what I need currently, although I also have several other software running to do other automation stuff. Many others I know like PowerHome.
Much like this (Carputers) hobby, lots of research, trial and error, money, time, and effort is required to get an X-10 home automation system to do everything you want it to. The X-10.com site has simple tutorials for the beginner. JohnWPB says CocoonTech is a one stop shop for info on home automation akin to MP3Car. I've also found HomeSeer.com a good resource.
Enough from me, have fun.