From Wikipedia: A door is a movable structure used to close off an entrance, typically consisting of a panel that swings on hinges or that slides or rotates inside of a space.
When open, they admit ventilation and light. The door is used to control the physical atmosphere within a space by enclosing the air drafts, so that interiors may be more effectively heated or cooled. Doors are significant in preventing the spread of fire. They also act as a barrier to noise.
They are also used to screen areas of a building for aesthetics, keeping formal and utility areas separate. Doors also have an aesthetic role in creating an impression of what lies beyond. Doors are often symbolically endowed with ritual purposes, and the guarding or receiving of the keys to a door, or being granted access to a door can have special significance. Similarly, doors and doorways frequently appear in metaphorical or allegorical situations, literature and the arts, often as a portent of change.