Types of rooftop structures

Belfry – A rooftop structure, or portion of a tower or turret, in which bells are hung.

Belvedere – A rooftop structure, or top level of a tower, accessible by stairs or ladder and from which one can look out. A belvedere has a roof and is open on one or more sides. The term means “beautiful view.”

Cupola – Cupola means “small cup” and is an architectural feature that resembles a small cup turned upside down. A cupola frequently crowns a roof, dome, or turret. In common usage, cupola is used today to refer to round, square, open, closed, occupied, and unoccupied structures.

Dome – A circular or spherical rooftop structure, though a spherical ceiling is also known as a dome. As an architectural feature, domes come in all sizes and shapes: onion domes, bell domes, saucer domes, etc. Sometimes, “dome” can refer to a cathedral: the Florence Cathedral in Italy is known as the Duomo.

Dormer – A structure projecting from a roof, usually containing a window.

Gazebo – While we often think of a gazebo as being a garden structure, a gazebo is also an ornamental rooftop structure open on all sides.

Lantern – A small structure, either open or with windows, crowning a roof. While it can be decorative, the primary function is to assist with ventilation or provide natural lighting.

Parapet – A low wall along the edge of a roof. Parapets were built originally to protect soldiers, but today are mostly decorative.Spire – A decorative element atop a roof, tower or steeple. It is typically narrow, tapered, and/or pointed.

Steeple – A tall structure frequently topped by a spire. In general usage, any tower attached to a church is referred to as a steeple.

Tower – A structure of great height when compared with its horizontal dimensions. It may be attached to a building or stand alone, and is typically taller than the structures around it. A tower may have a roof or be open on the top level.

Turret – A tower-like structure attached to a larger building and beginning above the ground level. Turrets are often ornamental and cylindrical in shape and typically have roofs.

Widow’s Walk – Also known as a captain’s walk, a widow’s walk is a flat roof deck or elevated platform, enclosed by a railing, from which one can look out. Widow’s walks are often found on truncated roofs (think of a sloped roof that stops abruptly and becomes flat). In legend, the wives of seafaring men would await their return while standing on the roof; alas, sometimes the men didn’t return, leaving widows standing alone.

From http://www.yorkblog.com/window/2008/03/of-cupolas-domes-towers-turret.html