Their modules have essentially become an industry standard. Radio Shack, IBM, RCA and X-10 themselves are just some of the companies making compatible modules. Basically, there are a variety of modules you can plug into an outlet. Then you plug whatever item, a lamp or appliance perhaps, into the module. You then plug a controller into another outlet somewhere else in your home and by pressing a button on the controller you can control the item.
Many modules allow dimming or controlling the device on a timer.
Check back here for automation projects and stories from other people. X-10 modules work using your existing lines in your home and all homes are wired a little differently. Once you get the set try varying the location of both the item you want to control and the controller itself. That way you can be sure you can expand your system in the future. That is, the controller sends a signal to a module, and that's it.