Just what you think it is. It's an oven fired by coal. The intense heat (usually around 800°F) that these things generate makes it possible to achieve a crisp crust with good oven spring. Pies cook so quickly in this heat that the crust doesn't have time to dry out, so it remains chewy and pliable...a by-product of the process is a fine charring of the crust, which many pizza fans find pleasing. If you're new to coal-oven pizza, the pizzaiolo didn't burn your pie; it's supposed to be that way.