By using my voltmeter, I can see that terminals marked "R" on my thermostat are 24 V AC from a transformer, "G" is ground. "C" is presumably for a multi-stage type equipment since the directions on my simple thermostat say it isn't used. What are the "W" and "Y" terminals for? They must be basically the "on" and "off" wires for relays, but what's the distinction between them? Is one of them for "fan only"?