Short answers, pulled from the story.
Excalibur is the legendary sword of King Arthur, said to possess magical powers and to symbolize the rightful sovereignty of Britain. Its blade was described as capable of blinding enemies in battle, and its scabbard was said to prevent the wearer from ever bleeding to death from wounds received in combat. Merlin told Arthur that the scabbard was the greater treasure of the two.
The name Excalibur derives ultimately from the Welsh Caledfwlch, a compound of caled (hard or battle) and bwlch (breach or gap). Geoffrey of Monmouth Latinised it as Caliburnus around 1136, and the name passed through Old French forms including Caliburn and Escalibor before reaching the spelling Excalibur in later medieval literature.
In the earliest tradition, Arthur obtains Excalibur by pulling it from a sword embedded in an anvil on top of a stone, as described in Robert de Boron's Merlin around 1200. In the later Post-Vulgate Cycle, a separate sword called Excalibur is given to Arthur by the Lady of the Lake after his first sword is shattered in a duel with King Pellinore.
In the Vulgate Mort Artu, Arthur orders his knight Griflet to throw Excalibur into a lake as he lies dying. Griflet fails twice out of reluctance before finally obeying, at which point a hand rises from the water to catch the sword. In the English Arthurian tradition, this role was transferred to Bedivere.
The scabbard of Excalibur was stolen by Morgan le Fay in revenge for Arthur's killing of her beloved Accolon. During her flight from Arthur, Morgan threw the scabbard into a deep lake, where it was lost. In Malory's Le Morte d'Arthur the scabbard is never recovered, though the Post-Vulgate says a fay named Marsique later found it and lent it briefly to Gawain.
On the 6th of March 1191, following the Treaty of Messina, the English king Richard I gave a sword identified as Caliburn (Excalibur) to Tancred, King of Sicily, as a diplomatic gift. The sword was reportedly discovered during the exhumation of Arthur's purported grave at Glastonbury Abbey in 1191, and its presentation was part of a broader pattern of Arthurian symbolism used by Anglo-Norman monarchs.