Yes, howstuffworks.com is great for an explanation on how a turbo works.

Most smaller turbos have an actuator wastegate. The wastegate controls how much boost you create. They have a "stock boost" level, which is usually about 3 or 4 psi. That means that without a boost controller, you will run that much boost.

What happens is when you create so much boost, it causes the actuator to open up and dump the exhaust pressure. You will keep running a specific boost level, but the excess pressure will dump off and not allow the turbo to create any more pressure.

With a boost controller, you can control when the wastegate opens and vents the excess pressure. They make both manual (which is usually a knob that you run vacuum lines to) and electronic (which works off of a valve and then runs wiring to the electronic unit).

If you need a better explanation just ask.

hih