The Room of Requirement is a secret room within Hogwarts that only appears when someone is in great need. Also known as the Come and Go Room and the Room of Hidden Things, it is located on the seventh floor opposite a tapestry of ballet dancing trolls. For the hidden entrance to appear you must walk past the wall three times whilst thinking about what you need. The room then transforms in shape, size and contents to fulfill your desires. It has been used by students, headmasters and house elves and has previously become a bathroom, living quarters and a passageway beyond the school grounds.
(Source: chrrycola, via wrightwatson)