Tuning is a process of altering fuel delievery and ignition timing to optimize performance for a given car, whether it be to achieve maximum horsepower or maximum fuel economy. Tuning is a very generic term that is often used without any understanding as to what it truely means. This confusion can probably stem from the fact that there are many ways to tune a car, and the methods for tuning will depend on the car itself. For instance, a carburated small block Chevy can be tuned with a handfull of springs and a phillips screwdriver for altering carburator calibration and distributor timing calibration, where as a modern fuel injected car would require that the ECU be reprogramed, unless additional electronics are used.
The degree of tuning necessary for a given car depends on the degree of alteration to the car. A stock car or a car with your basic bolt-on modifications will not need to be tuned to run properly, but as the degree of modifications increases, tuning begins to become a necessity. Just about all stock production cars are tuned for reliability, reduced emmissions and fuel economy, leaving room for small improvement on even a stock car for increasing power. Primarily tuning comes into play when the motor has been radically altered. Modifications that will generally necessitate a tuning are cam changes, cylinder head modifications, bottom-end changes, and force induction. With a highly modified motor, significant gains can be seen from properly tuning the motor.
In this day and age of fuel injection, there are alot of options that will allow an enthusiast to tune his car:
1) ECU Reprogram - You can have the ECU reprogrammed. This is a process of altering the fuel and ignition timing maps that are stored in the "brain" of the car. Unfortunately will most likely require shipping your ECU off to someone else. In order for the to properly tune it to your set up and modification, you need to provide them with as much information as possible. The best data to provide is air-to-fuel ratio data throughout the entire rpm range. This may be an iterative process too, in which you have to send the ECU off a second time for some fine tuning.
2) Piggy-back Electronics - These electronic devices take on many shapes, sizes and colors, but the basic principle is the same. These devices intercept one or more sensors on your car and alter the signal to give false readings to the ECU of the car. The ECU will in turn alter the fuel delievery and ignition timing based on the altered signal. This is a very effective method for moderate levels of tuning and this gives the user control of the tuning as well. This is a very attractive option for people on a tight budget and are not running a radical motor.
3) Stand-alone ECU's - These electronics devices also take on my shapes, sizes and colors, giving the customer alot of options to choose from. Basically a stand alone ECU replaces the job of your stock ECU in controlling the motor. This is the ultimate solution as it gives the user full control over the motor, so it can be tuned for any possible configuration. The downside as the cost of these "big-boy toys" can easily cost over a $1000 to as much as $5000 for parts alone. Installation is often tedious and requires a working knowledge of modern cars or be prepared to shell out at hundreds of dollars for installation.