From Academic Kids
The vocal cords, also known as vocal folds, are composed of twin infoldings of mucous membrane stretched horizontally across the human larynx. They vibrate, modulating the flow of air being expelled from the lungs during phonation.
The folds vibrate when they are closed to obstruct the airflow through the glottis, the space between the folds: they are forced open by increased air pressure in the lungs, and closed again as the air rushes past the folds, lowering the pressure (Bernoulli's principle). A person's voice pitch is determined by the resonant frequency of the vocal folds. In an adult male this frequency averages about 125 Hz, adult females around 210, in children the frequency is over 300 Hz.