Tuning 101

This post may be oversimplified for car enthusiasts, but it's meant as a brief description of what 'tuning' is for those who have never heard of it.

Before we begin:
Remember that more air in the engine equals more power.
Atmospheric pressure is roughly 14.7 PSI.
A normal engine can not take in any moire air than atmospheric pressure will allow.
These are called naturally aspirated engines, or N/A.

Now, there are two devices that can force more air into the engine, turbochargers and superchargers.
If your engine is equipped with either of these, it has quite a bit more potential. The extra air pressure created is called 'boost'.
Superchargers are more mechanical, run by a belt.
Here we will focus on the turbocharger.

The engine's brain:
The engine is controlled by a little computer called an ECM, or engine control module.
This runs software written by the manufacturer that controls how much boost your car makes.

What tuning is:
It's possible to rewrite this software, and make a turbo produce more pressure.
It's a delicate process, as every one change requires another to maintain the proper fuel/air ratios for safe engine operation.
The potential can vary from a small increase in power to incredible increases, depending on how strong the internal parts of the engine are and how much it can handle before breaking something.
It's important to note that many factory turboed engines are quite limited by their manufactures, for many reasons (in order to not compete with their more expensive models, or in order to keep fuel economy high, among other reasons). Unlocking this potential, in a nutshell, is what engine tuning is all about.