Wired GPS Receivers basically consist of a GPS Receiver, which looks similar to a PC mouse, which is powered by the car battery via a car accessory plug. This plug will not only power the GPS Receiver, but the PDA too using a Y-Splitter that is sold with the GPS Receiver.
Wired GPS Receivers are great if you have a Pocket PC or Palm and primarily want to have a GPS in-car. However if you want to take a receiver out of the car and use for walking or hiking, the wired GPS Receivers are not really practical, and you should look at a PDA GPS Sleeve/CF Card GPS Receiver, or a Handheld GPS Receiver