The wastegate is what controls the level of boost the turbo generates, if you look at the exhaust side of the turbo theres the exhaust wheel and the hole with the flap over it, that hole is the wastegate of the internal type. The boost is controlled by the actuator holding the wastegate flap closed until a set pressure is reached where the flap is opened so the exhaust gas bypasses the turbine wheel, the actuator then modulates the amount the flap is opened by to control and hold the boost level. An external type wastegate is generally used when running high boost levels or when more precise control over the boost is required. With these the internal wastegate is welded shut and an external wastegate is piped off from the exhaust manifold with the boost level controlled by a spring in the external wastegate. A dump valve is used to release the pressure from the intercooler piping when the throttle body is closed and either has a diaphragm or spring which sets the pressure at which it opens or dumps the pressurised air from the pipework.