Pre wired means it has the plug for the wall outlet and the power strip to plg your heat mat into already wired to the thermostat. Non wired means you are just getting the thermostat and the probe and thats it. You then have to wire an extension cord to your themostat and wire the power strip to the thermostat when you get it. Hope that helps...
Edit - You have to buy the extension cord and power strip your self if you do not get it pre wired also. The thermostat will not come with it.