Thermostat wires are the wires that run from the thermostat to the furance. These are usually behind the walls. If you have air conditioning, there are usually 4 wires. There are also two thermostat wires that run out from the thermostat to the condenser. The wires are usally inside of a casing and each wire is insulated from touching the other wires. These wires are very small in diameter. They handle 24VAC.

There is a standard color code for the wires that go to the furnace and thermostat:
R-red wire for power
W-white wire for the heat
Y-yellow or blue wire for the air conditioning
G-green wire for the main fan

There is no color code for the thermostat wires that go to the condenser.  This is usually a red and white set of wires.

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