EFI FUEL TUNING STUFF
Fuel MAP
This is a grid with engine speed on one side and engine load on the other. The MAP or MAF sensor signal tells us engine load and, based on this and the engine speed, the ECU looks up the two coordinates on the fuel map and injects the correct amount of fuel. Programmable ECUs allow you to change these values and change the AFRs throughout the operating range on the engine.
Oxygen Sensor
This one's a biggie. This sensor is the device that tells us the AFR. Without it, you can't effectively tune. Mounted in the exhaust stream, it measures the amount of oxygen leaving the combustion chambers and is used by the computer to calculate the AFR. There are two types of O2 sensors: narrow and wideband. Unless you live in the third-world or you like inaccurate data and crappy tunes, get the wideband. It's faster and has a useable AFR range (10:1 to 20:10), while the narrowband works well only around stoich (14:1 to 15:1). If $200 is still too much for the cheapest of ya'll, there's always the DIY wideband O2.
Rising Rate Fuel Pressure Regulator
Used primarily with low-dollar and smog-legal forced induction kits, this device does nothing under manifold vacuum conditions, but once the engine sees boost, it increases the fuel pressure proportionally with boost pressure. The increased pressure at the fuel injectors means that more fuel gets into the engine. It only works with return-type fuel systems and since it does not change the signal to the injector, you can't fine-tune it.
Injector Signal Modifiers/Generators
For simplicity, we've grouped together all the electronic devices that change or replace the signal to the fuel injectors; including ECUs, AFCs, piggybacks, re-flashes, etc. What these devices do is control how much fuel is delivered to the engine. They typically have their own displays or link to a laptop computer to allow the user to add or subtract fuel according to the wideband in order to hit their AFR targets in all the different "zones" on the fuel map. The fancier ones have features that allow it to communicate with the oxygen sensor and self-tune the AFR for all the different engine loads and speed all by themselves--while you drive.
Extra Injector(s)
Just what it sounds like. Mounted somewhere in the intake tract (usually near the throttle body), the extra injectors are controlled by an extra computer. A downside to these units is that fuel can only be added, not subtracted from the base map.
MAP/MAF Sensor
Most fuel-injected cars have either one of these sensors. This is the signal that will tell you the engine load. Learning to interpret this signal is the key to getting a good tune. Depending on the load, you need to vary the AFR. The MAP (manifold-absolute-pressure) sensor is mounted on the intake manifold and tells the computer how much pressure or vacuum is in the manifold. Based on engine speed and load (MAP), the total airflow can be calculated. MAF (mass-air-flow) sensors measure airflow directly.