A throttle body is merely a tube with a flat circular plate that pivots to limit airflow into the intake manifold. As you apply throttle, the plate opens to admit more air through the tube; this causes the air mass meter to sense greater airflow and signal the ECU to increase fuel injector action, all of which accelerates the engine. The throttle body has higher vacuum on the engine side of the plate, which is why vacuum supplies run off that side to various components. To control idle when the plate is fully closed, the engine receives metered air through a bypass hose from the idle air control motor, which valves air from the upwind side to the downwind side around teh closed plate to maintain constant idle speed. This motor is turned on when the ECU receives an "idle" signal from the throttle position sensor, mounted to the side of the throttle body.

If your throttle body is full of junk, the idle will be affected by improper closure of the metering plate. That's why it's important to clean it out every so often.