Officially defined by good 'ole Merriam-Webster, a cupola is simply "a small structure built on top of a roof." The Penguin Dictionary of Architecture defines a cupola as "a dome, especially a small dome on a circular or polygonal base crowning a roof or turret." However, these days, people have used the term "cupola" to define structures that aren't necessarily dome shaped.
Here are a few examples of Visbeen-designed homes with cupola's perched atop their roof...
There is another type of roof-type structure that has often been grouped under the term cupola but is actually a little different. A monitor is a raised structure running along the ridge of a double-pitched roof, with it's own roof running parallel with the main roof. The long sides of monitors usually contain windows. It has been said that these structures were named monitors because they were often used on the rooflines of larger estates, especially lake or oceanfront residences, so that someone could monitor what was going on around the home from an aerial location. (I'm hearing "The British are coming! The British are coming!" in my mind right now. Maybe Paul Revere wouldn't have needed the midnight ride if he had a nice monitor to sit in. Just a thought.)
So, "queue-pola" or "coo-pola?" Either way, they add a bit of architectural interest and a lot of whimsy to the roofline of a variety of home styles.