You've seen these before. You open the door once, and it's a lavatory. You open it again and it's a closet.
A Door of Many Rooms works on two levels. Before entering a DoMR, unless you are registered as a party, each person opening a DoMR opens a portal to a different instanced copy of the room behind the door. The room behind the door is often set up as a sitting room. Once inside, the DoMR functions much like a Room of Requirement: need to piss, open the door and find a urinal; tired, open the door to a bedroom; ready to leave, open the door to leave the way you came. The available rooms are designated and furnished by the owner.
Anyone wanting to use a DoMR needs to be registered to the door, otherwise, they'll just have access to the sitting room. As long as you are registered to a DoMR, any items you leave in the "room" will stay there. However, upon checking out(un-registering), all items left in the DoMR will be transferred to a storage room accessible to the DoMR's owner.
Mechanics & Inner Workings
It is a solid wooden door with metal bands and a polished metal handle with a thumb-pad. The metal bands are inscribed with numerous runic characters. They register your thumbprint at a kiosk at the front desk. This allows you to open the DoMR.