It is a structural extension which projects vertically from the plane of a sloping roof. It usually contains Juliet balcony and windows. Dormers are commonly used to increase the usable space in a loft and to create window openings in a roof plane.
On the inside, what may be dark, attic space can become habitable with a dormer window. An additional bathroom can be enlarged by a dormer tucked into a large bedroom. Besides additional space for a home, natural light and ventilation can make interiors more inviting and healthier.