The term 'roof window' can refer to two different types of windows. The first is a DORMER, which is a window that projects vertically from a sloping roof. It typically has its own roof, and is used to add light and ventilation to an attic or upper floor. The second type of roof window is a SKYLIGHT, which is a window installed in a roof to allow natural light into a building. Skylights are often used to brighten up dark rooms or hallways.