well, a control panel is something you'll find in your control panels folder, and can be either a "control panel" or an application. basically, it adds functionality to your machine while letting you alter some of the added functionality's settings.
extensions on the other hand, do pretty much the same thing as control panels, insofar as they add functionality. however, they don't provide a way of tinkering with the settings. you can either add the function by putting it in the folder or remove it by de-installing the extensions.
so, it's not really that anything in your system folder can be called a control panel, but that different things in your system folder do essentially the same sort of thing.
-r.