Vocal Cords
Folds that're placed directly in front of the lungs (before the larynx). These flaps rapidly open and close to produce small pulses of air from the air-pressure exiting the lungs.
This rapid opening and closing is often termed the Bernoulli affect.
The rate of this oscillation is called the fundamental frequency (\(F_x\) or \(F_0\)) and influences the pitch of the voice.
Mechanics
The acoustic output from this process doesn't result in a pure tone, it's more of a hard edged [see page 5, graph]. As the glottis opens gradually, from the pressure build up, we see a slight increase in air pressure. Then it quickly slaps back together causing a sudden decrease in pressure.
This rapid hard edged wave form produces a buzzy sound which can be shaped. If it was just a simple sine wave then all the other parts of the vocal system could do was is it louder or quieter.