Doors are the entrances to The Empty City. They can appear at any place at any point in time, even over other doors. Some Doors have been described as elaborately ornate. Others have been described as normal and plain.
Drawing Doors on flat surfaces may prevent them from appearing. In To Light a Candle, a patient at Shady Lawn would constantly draw Doors on the walls with chalk until he was stopped, at which point an actual Door appeared. It is not known if the drawings were actually preventing the Door from appearing or it just chose that time to appear.
The Rapture Cycle's version of the Doors are called rabbit holes leading to Xanadu. They go way beyond Doors, however, sometimes engulfing entire cities and countries.