Why does a cell phone charger continue to consume power when plugged into an outlet but not the phone?

Most electronics will consume a little power even if they are off or not in use. In the case of a cell phone charger, you may notice that if it is plugged into the wall, but there is no cell phone charging, the charger will still be warm. I don't know the physics behind that, but anything that is plugged in will take up a little energy, even when not in use.