The Round Table was the table around which King Arthur's knights met. It has survived into the present day, though its whereabouts are as yet unknown.
Real World Background
The Round Table is first mentioned in the Roman de Brut, a 12th century Norman-French Arthurian pseudo-chronicle written by Robert Wace. Wace says of it that King Arthur's knights assembled around it, and that the table's roundness kept them from quarreling over precedence; he describes it as famous among the Britons, suggesting that it might have appeared in oral legend before his time. Layamon's Brut, an Anglo-Saxon adaptation of Wace's work, expands on this with an account of how, during one of Arthur's court feasts, the knights and nobles quarreled with each other over who was to sit near the head of the table; the quarrel led to a brawl in which many were slain, until Arthur put a stop to the fight. Afterwards, to prevent further such quarrels, he commissioned a skilled carpenter from Cornwall to build a round table, where his knights could sit as equals.
The early 13th century French romancer Robert de Boron gave a more mystical interpretation of the Round Table and its origins in his verse romance Merlin. Here, Merlin makes the Round Table for Uther Pendragon as a spiritual descendant of the table of the Last Supper and the Grail Table (where Joseph of Arimathea and his followers held the feast of the Holy Grail); instead of being made for all the knights at court, it is meant for a select few. The Vulgate Cycle expanded on this account (followed by Sir Thomas Malory), stating that after Uther Pendragon's death, the Round Table passed to King Leodegrance of Cameliard, Guinevere's father. When Arthur married Guinevere, Leodegrance gave him the Round Table as a wedding present. Each Pentecost, the knights of the Round Table would meet at Arthur's court and renew the vows that they had taken when admitted to the order: to never fight in a wrong quarrel, to always grant mercy to defeated enemies, and to always come to the aid of ladies and damsels in need.
One seat at the Round Table was the Siege Perilous, reserved for the knight who was to achieve the Holy Grail (usually identified as Sir Galahad, though some early accounts give the role to Percival). Anyone else who sat in it would be consumed by fire.
The Round Table's fate is not recorded in Malory; the Post-Vulgate Cycle claims that it was destroyed by King Mark of Cornwall after Arthur's death (obviously not true in the Gargoyles Universe).