Older systems and base signals on newer ones 
Terminal  Probable
wire Color  Signal  Description 
C  Black  24 Vac Common  From one side of the 24Vac transformer (Think of this as 24Vac neutral 
R or V  Red  24 Vac Power to be switched  From other side of the 24Vac transformer (Think of this as 24Vac L1) 
Rh or 4  Red  24 Vac Heat call switch power  Same as R, but dedicated to the Heat call switch 
Rc  Red  24 Vac Cooling call switch power  Same as R, but dedicated to the cooling call switch 
G  Green  Fan  Fan switch on thermostat - is connected to R when Fan /Auto switch is in the fan position 
W  White  Heating call  Is connected to R or Rh when themostat calls for heat. (Might be jumpered to Y on a heat pump. On others might be second stage heating 
Y  Yellow  Cooling call  Is connected to R or Rc when themostat calls for cooling. Also Cooling or 1st stage heating on a heat pump
Most often connected to G when Fan switch is set to auto
 
New signals - for heat pumps and staged systems 
Terminal  Probable
wire Color  Signal  Description 
Y2  Blue or Orange  2nd stage Cooling  
W2  Varies  Second Stage Heating  First stage Auxiliary heating on a heat Pump 
E  Varies,
blue, pink,
gray, tan  Emergency heat relay on a heat pump. Active all the time when selected, usually not used.  Disable the heat pump and turn on first stage Aux heating 
O  Varies,
Orange  Reversing valve  Energize to cool (Changes from heat to cool on heat pumps) 
B  Varies,
blue, black
brown, orange

 Sometimes common side of transformer. Needed on some electronic thermostats or if you have indicator lamps
OR
Reversing valve (energize to heat) above. York and Trane sometimes use (B) as common.  Might be heating changeover or on older stuff common of transformer. 
X  Varies  SEE NOTES BELOW  Might be common but sometimes emergency heat relay 
X2  Varies  Second stage heating or indicator lights on some thermostats  Might be emergency heat relay 
T  Varies,
Tan or Gray  Outdoor anticipator reset  Used on GE/Trane/American Standard and some Carrier Products. 
L  Varies  Service Light