Go to
www.howstuffworks.com and look up wastegate and boost controller.
wastegate is on the exhaust manifold and opens the dump tube once the spring is under enough pressure(boost) to open to prevent more boost from building and passing through the turbine.
a boost controller can add to the pressure required to open the dump tube
A boost controller allows you to run more boost than the wastegate spring would allow.
Wastegate is required, boost controller is not.