Meaning of "W" and "Y" on thermostat connections?

[Stephen Tashiro]

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"?

[ray hampton]

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"?
G is green,B is black or blue,R is red, W is white , Y is yellow according to the junction boxes that I wired
C is must likely short -hand for common

[Joe Leigh]

R= hot leg of the 24v ac transformer.
C= common leg of 24v ac transformer.
Put an ac volt meter between R and C and you will read 24v ac. Most residential thermostats do not require a C or common leg at the thermostat

W=the heat load. On a call for heating the thermostat closes a switch between R and W, sending 24v to energize the heating circuit.
Y= the cool load. On a call for cooling the thermostat closes a switch between R and Y, sending 24v to energize the cooling circuit.
G= the fan load. On a call for cooling the thermostat closes switches between R and Y and also R and G energizing both cooling and fan cicuits.

The fan circuit can also be energized by switching the "fan" switch from auto to on if your thermostat has one. Depending on the type of thermostat you have, the thermostat cam also be programed to energize G (the fan circuit) on a call for heating.