A simple AFC (converter not controller...) can get your feet wet into tuning. The AFC will "trick" your ECU into thinking more air is coming in (or less) and to compensate the ECU will pulse the injectors to release more fuel. If it "tricks" the ECU to think less air is coming in, the injector pulse is shorter and less fuel will be delivered. Changing these variables anywhere outside of 14.7 a/f is making the car "rich" or "lean".
A rich mixture is more stable but more energy is wasted because all the fuel is not combusted. On the contrary, a lean mixture makes the most use of the given fuel but can be very unstable. The compression ratio is a realative figure that illustrates the amount of pressure on the mixture of a/f before it is ignited by the spark plug.
Changing spark plugs is usually to meet one goal: their operating temperature. To run "colder" plugs means that the plug will not stay as hot and thus reduce against detonation (combustion before the spark actually ignites due to temperature in the combustion chamber and/or an overly lean/unstable mixture).
ECU tuning will usually have to do with getting more timing out of the engine. By this I am referring to the relative time between the intake valves opening, letting air into the combustion chamber to be compressed by the piston, spark plug igniting, and exhaust valves opening. As you alter the mixture and how fast/how intense the mixture exploads, the mechanical components need to be able to keep up (or slow down.)
Cliff's: A basic AFC should get you started.
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