A dormer is a component or structural part of a building, usually one that comes out of the plane of a roof surface. When a roof is sloping downward toward the street, for instance, a dormer will come out of that slope in the roof with the window facing the street.
The roof of the dormer will be perpendicular to the roof itself. In loft conversions, or to add more room to a house without raising the entire roof and building a complete second floor, a dormer will often serve the purpose of giving the homeowner an extra room or two, or more light in a top story room.
A blind dormer is a dormer which is put onto the exterior of a house for mere show or esthetic effect, and which has no actual functional value. The dormer itself will be boarded up underneath and there will be no access to it from the house. These blind dormers are often added to large houses in order to make them seem even more impressive.