I post this somplace every few years. I havn't done it in a while, so here goes:
(R), Red conductor, hot side of transformer
(C) or (B) or (X), Blue, Black or Brown conductor, Common side of transformer. Not to be confused with (B) on heat pumps. If you have a (B) on a heat pump thermostat, your common will be (C) or (X)
(Y), Yellow conductor, 1st stage compressor contactor (cooling for ac or cooling and heating on a heat pump).
(Y2), Blue or Pink, second stage compressor contactor
(W), White conductor, 1st stage heat (gas furnace, oil furnace, electric resistance furnace (auxillary heat on a heat pump)
(W2), Pink or any unused color conductor, second stage of heat
(G), Green conductor, furnace blower fan.
(O), Orange conductor, Energize heat pump reversing valve to cool
(B), Blue or Orange, Energize heat pump reversing valve to heat (mainly older Rheem/Ruud).
(E), blue, pink, gray or tan conductor, emergency heating strips on a heat pump.
(T), Tan or Gray conductor, outdoor temperature compensator.
(L), Blue, brown, tan or gray conductor, service indicator lamp.
Keep in mind that not all systems use all these.