How it works
A buzzer (specifically a piezoelectric buzzer) converts an electrical signal into an audible tone. It contains a small piezo-ceramic crystal bonded to a thin metal disk. When a voltage is applied, the crystal slightly changes shape. By applying an oscillating (AC or PWM) voltage, the crystal vibrates rapidly, producing sound waves.
Active buzzers contain their own internal oscillator and will sound a continuous tone just by applying a DC voltage. Passive buzzers (like the one modeled here) require you to provide the oscillating frequency (e.g., from a 555 timer) to produce a pitch.