"Sometimes it is there, and sometimes it is not..."
The Room of Requirement is a magical room in Hogwarts castle which can only be discovered by someone who is in need. It is called the Come and Go Room by the house-elves of Hogwarts. The Room is located on the seventh floor, opposite a tapestry showing Barnabas the Barmy trying to teach trolls to dance the ballet. To make the Room appear, a person has to walk past the section of blank wall three times concentrating hard on what is needed.
In 1927, Albus Dumbledore visited the Room of Requirement where the Mirror of Erised was stored at that time. At the time it was a spartan room with the Mirror covered in black velvet (CG73).
Dumbledore once happened upon the Room of Requirement early one morning when his bladder was especially full. When he found it, it was full of chamber pots. (GF23)
Fred and George once needed a place to hide from Filch and the Room became a broom cupboard.
Filch sometimes finds an extra supply of cleaning products in the Room when he's running short (OP18).
Dobby brings Winky there to sleep it off after a particularly nasty bout of butterbeer; in those cases the room has held antidotes to butterbeer and a "nice elf-sized bed" (OP18).
Harry found a room ready for teaching Defence Against the Dark Arts lessons. It contains bookcases, cushions, and even, when Harry suddenly realized he needed one, a whistle (OP18). Harry discovered, just before Christmas, that Dobby had decorated the room with Christmas ornaments bearing Harry's face and the words "Have a Very Harry Christmas!" The house-elf had also hung mistletoe in the middle of the room, and after the D.A. meeting, Harry had his first kiss under it (OP21).
Dobby reports to Harry that Draco has been using the room. (HBP21) Harry realizes that it has never appeared on the Marauder's Map, and Hermione surmises it is Unplottable. It is impossible for Dobby to follow Draco inside, or for Harry to get in without knowing what purpose the room is serving at the time.
Desperate for a place to hide the Half-Blood Prince's copy of Advanced Potion-Making, the Room became the size of a large cathedral, with high windows. It looked like "a city with towering walls", full of objects hidden by generations of Hogwarts inhabitants. (HBP24)
Items Harry noticed include:
broken and damaged furniture
thousands and thousands of books
chipped bottles of congealed potions
what looked like dragon eggshells
corked bottles whose contents still shimmered evilly
several rusting swords
a heavy, bloodstained axe.
an enormous stuffed troll,
the broken Vanishing Cabinet (in which Montague had got lost the previous year (OP28), and which Draco Malfoy repairs and uses to allow Death Eaters into Hogwarts by way of its twin at Borgin & Burkes (HBP27))
Harry hid the Book in a large cupboard that's surface seemed to be blistered by acid; behind a cage which held a skeleton of a creature with five legs.
To mark the location where he hid the book, he placed a bust of an ugly old warlock on top of the cupboard, perched a dusty old wig and a tarnished tiara on the statue's head (HBP24). This tiara, as it turned out, was the Diadem of Ravenclaw, hidden there by Tom Riddle in 1965 (HBP20, DH31).
Sibyll Trelawney uses the Room of Requirement to hide her empty sherry bottles. On one of her visits she encountered the Room occupied by Draco Malfoy who had just successfully fixed the Vanishing cabinet. He used Peruvian Instant Darkness Powder and threw her out of the Room, where Harry was in the corridor. (HBP25)
The room converted itself into a makeshift dormitory for the students at Hogwarts who were fighting back against the Death Eaters and needed a sanctuary (DH29).
Harry, Ron, and Hermione entered the Room of Requirement in its warehouse form to find the Diadem of Ravenclaw. They had just gotten their hands on it when Malfoy, Crabbe, and Goyle accosted them. Crabbe set the room ablaze with Fiendfyre, which destroyed the Diadem and killed Crabbe. The others managed to escape (DH31).