A door is a moving mechanism used to block off and allow access to an entrance to a building, room or vehicle. Doors normally consist of one or two solid panels, with or without windows, that swing on hinges horizontally. These hinges are attached to the door's edge but there are also doors that slide, fold or spin. The main purpose of a door is to control access.

Doors are significant in preventing the spread of fire and act as a barrier to noise. Many doors are equipped with locks to allow entrance to certain people and keep out others.

When opened, doors can allow almost anything to pass through. 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. When closed, a door normally impedes the transfer of air from one side to the other. Similar structures that do allow air to be transferred through some form of a grillwork are called gates.

Doors may have an aesthetic purpose in creating an impression of what lies beyond; for example, keeping administrative and factory areas of a building separate. In less formal settings, doors may also be seen as a sign of the desire for privacy. As a form of courtesy and civility, people often knock before opening a door and entering a room. Some doors even have designated "knockers." 

Doors are often symbolically endowed with ritualistic purposes. For example, being granted access to a door, including the guarding or receiving of the key to that door, may have special significance. Similarly, doors and doorways frequently appear in literature and the arts in metaphorical or allegorical context, often as a portent of change.

Credit to Wikipedia for great information on doors!