Thermostats are wired to perform several functions. Each wire connects to a terminal that controls different variables of the heating and/or air conditioning system. Honeywell thermostats normally have four colored wires in their configuration.
This wire connects the thermostat to the transformer that controls both the heating and cooling functions. Unlike the other wires that control specific relays, this is the controlling wire for all operations.
The green wire connects to the G thermostat terminal whose function is to either control the system's fan or serve as a relay to the blower controls.
The air conditioning part of the heating/cooling system is controlled by the yellow wire's connection to the Y terminal.
The white wire controls the heat generated by the system and it connects to the W thermostat terminal.
These four wire colors are standard on most Honeywell thermostats but older or different styles -- or ones that have been previously been installed or repaired -- may have wires of different colors. Consult a professional with wiring questions to avoid damaging the thermostat or heating/cooling system.