How do dynamic headphones work ?

How do dynamic (or electrostatic or electret or 4-channel or etc.) headphones work?

In the past few years, the available headphone technologies have both expanded and contracted. On the one hand, dynamic transducer headphones now dominate all headphone sales, with electrostatic headphones being the one remaining form of audiophile exotica. Competing transducer technologies, such as isodynamic and electret types, have virtually disappeared from the marketplace. On the other hand, there is great demand for noise-cancelling and surround-sound phones. New developments in headphone design may finally make virtual listening an affordable reality. For more information about how headphone technologies work, see A Quick Guide To Headphones and Technologies For Surround Sound Presentation in Headphones.