A cold air intake is a device used to bring down the temperature (http://en.wikipedia.org/wiki/Temperature) of the air (http://en.wikipedia.org/wiki/Air) going into a car (http://en.wikipedia.org/wiki/Automobile)'s internal-combustion engine (http://en.wikipedia.org/wiki/Internal-combustion_engine), to slightly increase engine power. A secondary goal is to increase the visual appeal of a car by changing the appearance of the engine bay. It may also slightly increase fuel economy, and can increase the life of the engine by allowing it to run cooler.
Standard air intake systems tend to be restrictive, in order to save under-hood space and minimize noise in the cabin of the car. Aftermarket (http://en.wikipedia.org/wiki/Aftermarket_(automotive)) cold air intake systems come in many different colors and sizes, and are easy to install when compared with other performance-increasing modifications

All cold air intakes operate on the principle of increasing the amount of oxygen (http://en.wikipedia.org/wiki/Oxygen) available for combustion (http://en.wikipedia.org/wiki/Combustion) with fuel (http://en.wikipedia.org/wiki/Fuel). Because cooler air has more density for a given volume, cold air intakes generally work by introducing cooler air from outside the hot engine bay. However, the term "cold air intake" is often used to describe other methods of increasing oxygen to an engine, which may even increase the temperature of the air coming into an engine.

Some strategies used in designing cold-air intakes are:

increasing the diameter of the air intake, allowing increased airflow.
smoothing the interior of the intake to reduce air resistance.
providing a more direct route to the air intake.
tuning the length of the intake to provide maximum airflow at certain engine speeds (RPM).
using a more efficient, less restricting air filter (http://en.wikipedia.org/wiki/Air_filter).

Some people think that a conical open element air filter in the engine bay connected to an aluminum tube going to the throttle body is a CAI. They couldn't be more wrong. Infact, I don't consider any intake with aluminum tubing to be a CAI. Aluminum heats up easily...especially when its as thin as it is on intake tubing. And when that happens it just gonna heat up the air going into the engine anyway...especially if the filter is exposed and under the hood. At that point the stock system is superior. Because they actually draw in air from outside the engine compartment AND they are made from material that dissipates heat very efficiently.