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Turbos create boost by spooling the impeller within the compressor side. The compressor impeller is driven by the exhaust impeller on the exhaust turbine, which is driven by the hot exhaust gases exiting the exhaust manifolds. The more exhaust energy there is, the faster the impellers spin. When the desired boost level is reached, the wastegate bleeds off some the exhaust gasses going to the turbine section to keep it from over boosting, with the extra exhaust either bled to the atmosphere, or refed back to the exhaust before the muffler. As boost level drops, the wastegate recloses just the right amount to modulate the right amount of boost pressure. Most cars have turbos sized to the engine which if not modulated by a wastegate, would create too much boost to the motor and destroy it from detonation. The wastegate is spring loaded, and controlled by the intake boost pressure. Intake boost fed to the wastegate, starts opening the wastegate valve at the spring pressure it is set at. Some factory turbo cars, intercept this boost signal to the wastegate with an electronic selenoid valve, controlled by a computer, to dial in boost up an down based on knock sensors and programming. Aftermarket computers such as an HKS EVC can dial boost from your cockpit with a push of a switch or turn of a knob. Cars without wastegates are few, and those that don't have them don't need them because the turbo is really undersized for the motor and already maxed out, incapable of creating excessive boost. They also create boost very slowly, so you don't get that instantaneous surge that comes when you floor the throttle. This is not a optimal solution, since undersized turbos are inefficient and limit the car's power. The inefficiency is converted to heat - hot air - with inefficient turbo systems being as inefficient as 65%. On the other hand, sizing the turbo larger, and controlling it with a wastegate, efficiencies as high as 78 to 80% are achieved.
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