The way it works is fairly simple. Water is placed in the bottom container of the machine and is heated. Once the water reaches the boiling point, it rises from the chamber and passes through a filter cup containing the coffee grinds. It then proceeds up a spout and collects as coffee in the upper chamber.
Here's what it looks like when it's coming through:
The Bialetti design has become an iconic image. While many different shapes have been developed for this type of stovetop appliance, the edgy Moka Express has endured relatively unchanged since it was invented. This type of coffeemaker is found in the vast majority of Italian homes.
The Bialetti company is now a large international firm.
For more information on the Moka Express, see here.
For more on Bialetti, see here.