Schematic diagram of central air conditioning

Several types out there. 99% of them use the same basic wiring.
Red is power from transformer to t'stat. Sometime there is a wire for common from transformer to t'stat too, but not always.
While is for heat. Green is for fan. Yellow is for cooling.
These 4 to 5 wires go on the terminals that are maked with letters: C=common, RH/RC is for power, W=heat, Y=cooling, G=fan.

In most cases, 2 wires from the indoor unit will go to the outdoor unit to power it for cooling. Usually connected to the yellow wired, Y terminal and common or grounded to the unit if the transformer is done in the same fashion.

If you have a heat pump, totally different animal and different wiring too.

Hope this helps you. Good Luck.