The Room of Requirement is a room in Hogwarts that only appears when a person is of in great need of it. The room then transforms itself into whatever the witch or wizard needs it to be at that moment. It is belived that the room is unplottable as it does not appear on the Marauder's Map, nor do its occupants, although this could simply be because James, Remus, Sirius, and Peter never found the room.
The room is located on the seventh floor of Hogwarts castle, and has a hidden entrance near a tapestry of trolls wearing tutus. The way to open the room is to walk past it three times thinking about what you need and the door will appear.
The first mention of this room is in Harry Potter and the Goblet of Fire when Albus Dumbledore tells Igor Karkaroff of his discovery of a room full of chamber pots when he needed to relieve himself, but was unable to find the room again later.
In Harry Potter and the Order of the Phoenix, Harry asks Dobby where Dumbledore's Army can safely meet. Dobby tells Harry of what the house-elves call the "Come and Go Room", or "Room of Requirement", and that he had used the room to treat Winky after she had gotten drunk on Butterbeer (during which it contained antidotes to butterbeer and an elf-sized bed for Winky to sleep of her drunkeness). He also says that Argus Filch had found extra cleaning supplies in the room. During the first D.A. meeting Fred and George Weasley revealed that it had once taken the form of a broom cupboard which they had used to hide from Filch.
Harry and Dumbledore's Army used the room as their headquarters and practice area. Among the items provided by the room for the D.A. were Dark Detectors and shelves of helpful books (incluing A Compendium of Common Cures, The Dark Arts Outsmarted, Self-Defensive Spellwork, and Jinxes for the Jinxed.
Harry used the room again in his sixth year to hide his copy of the Half-Blood Prince's Potions textbook from Snape. He found that many other students had used the room to hide a multitude of other objects: broken and damaged furniture (perhaps hidden to hide mishandled magic, or perhaps castle-proud house-elves, thousands and thousands of books (probably banned, graffitied, or stolen), winged catapults, Fanged Frisbees, chipped bottles of congealed potions, hats, jewels, cloaks, what looked like dragon eggshells, corked bottles whose contents still shimmered evilly, several rusting swords, a heavy, bloodstained axe, an enormous stuffed troll and the broken Vanishing Cabinet that Montague had gotten lost in during the previous school year. (In the cabinet he found a cage that contained the skeleton of something that had 5 legs.)
Also in Harry's sixth year, Professor Trelawney attemped to use the room to hide her sherry bottles, but Draco Malfoy threw her out. Draco was using the room to repair the Vanishing Cabinet so he could smuggle Death Eaters into the castle (via a matching Vanishing Cabinet in Borgin and Burkes).
In Harry Potter and the Deathly Hallows, Neville Longbottom and several other students use the room as a hideout to escape the wrath of the new Death Eater professors, the Carrows. The hideout connects to the Hog's Head, allowing members of the Order of the Phoenix to enter the school to help fight off Lord Voldemort's attack on the school.
Later that same year, Harry realizes that Rowena Ravenclaw's Diadem, one of Voldemort's Horcruxes, is hidden in the room, as he has seen it in his sixth year while hiding his Potions book. The trio manage to find it but are ambushed by Malfoy along with Vincent Crabbe and Gregory Goyle. Crabbe starts a huge fire (which Hermione believes is Fiendfyre) that destroys the Horcrux.
It is advised that the users of this room be very specific about what they are looking for, as it has been shown that other people can enter the room and see what the occupant is doing if they know how the room is being used. The Inquisitorial Squad finds the hideout of the D.A. in Harry Potter and the Order of the Phoenix by knowing the D.A. was in the room at the time. Neville uses this knowledge to create his hideout as he specifically asks that the hideout be made unavailble to those who support the Carrows.