Originally Posted by NeedsWings

the wastegate is what stops the turbo from creating too much boost by opening a diversion port in the exhaust to go around the turbo's exhaust wheel.

a blow off valve is there so that when you are shifting gears and you momentarily let the throttle plate close the pressurized air has a place to escape (out the blow off valve / pressure release valve) instead of being slammed into the closed throttle body and backfeeding the pressure to the turbo which wants to keep spinning forward. that is on a blow thru TB designed car where the inlet (compressor) side fo the turbo is BEFORE the throttle body