Back to basics...

Remarkably interesting....

"A door is a generally floor-length opening in a wall (or other partition), often equipped with a hinged or sliding panel which can be moved to leave the opening accessible, or to close it more or less securely. Doors are nearly universal in structures of all kinds (especially houses and other buildings), allowing passage between inside and outside, or among internal rooms. Doors are also found in vehicles, cupboards, cages, etc.
The purpose of a door opening is:
to allow people, animals and objects to pass
for ventilation
The purpose of a door closure is:
preventing passage of infiltration air; reducing air drafts and creating an enclosed space that can be heated or cooled more effectively (revolving doors are especially efficient for this purpose)
privacy, and noise reduction
keeping occupants inside a vehicle
regulation of access, especially when combined with various types of locks
for aesthetics (e.g. cupboard doors cutting off view of the contents)
to help prevent the spread of fire
Doors and doorways can also appear in metaphorical or mystical situations; for example, a spiritualist might speak of "opening a door to the other world". Doors can have ritual purposes (one example concerns the doorkeeping duties of the Gentleman Usher of the Black Rod).
The term door can refer either to the opening or to the movable panel or panels which closes it. The term doorway can be used to refer specifically to the opening. When framed in wood for snug fitting of a door, the doorway consists of two vertical jambs on either side, a lintel or head jamb at the top, and perhaps a threshold at the bottom. When a door has more than one movable panel, one of the panels may be called a leaf."

Taken from Wikipedia