This is the magical sword of Arthur. it's name comes form Geoffrey of Monmouth's Caliburn, whic comes from latin chalybs or steel. Sometimes it is referred to as the sword in the stone, other times ashaving originally been Gawain's.

Arthur removes the sword from the stone, proving to be the rightful heir to the throne of England. (2)
the tradition of Excalibur the name was first used for king arthurs sword by the French Romancers. it was Not the famous "sword in the stone", but actually a second sword that ws acquired by the king through the interseccion of merlin. Merlin took Arthur to a magical lake where "the lady of the lake" thrust her hand out of the water holding the magical sword. this sword was unbreakable and made by an avalonian elf smith, and along with it came a scabbard which would protect him for as long as he wore it. Excalibur was stolen near the end of king arthurs reign by his evil half sister, Morgan LeFay. the sword was found but the scabbard was lost forever, causing arthur to be mortally wounded at the battle of camlann. Arthur then instructed for the sword to be brought back to the lake in which it came from. (3)
Here is a picture of Arthur in a boat with Merlin, reaching out to take Excalibur from the Lady of the Lake.
"How Arthur Gat His Sword Excalibur" (4)

The Irish word for Excalibur was Caladbolg, which means
calad(hard) bolg(lightning).(5)
This is a screen shot from mists of avalon . This is when arthur is first given the sword, by vivane and it is first seen as a cross sybolizing the fact that camolot will eventually be run over with christians and that things arent always what they seem. arthur pulls the sword form the stone alter. (6)
Difference Between Excalibur and The Sword in the Stone
The Sword in the Stone was the first sword that Arthur rightfully earned when to show he was worthy. This though was only a sword showing that he would be King of England and was the rightfull heir to King Uther until it broke. The Lady of Lake then gave Arthur Excalibur which was to be unbrakable and the scabbard which would protect him from being mortally wounded. This sword, instead of saying he would be king, said that he would then on be accepted as the protector of Britain and was proven worthy. (7)
Arthur's sword was first called Caliburn but later writers changed it to Excalibur. Caliburn was given to Arthur by the Lady of Avalon
(8)
Sources
(1)
www.pantheon.org(2)
http://www.pittstate.edu/engl/nichols/arthur.html(3)
http://www.britannia.com/history/arthur/excalibur.html(4)
http://www.lib.rochester.edu/camelot/images/bkaexc.htm(5)
http://www.kingarthursknights.com/items/default.asp(6) mist of avalon by, marion zimmer bradley and
http://www.twoevilmonks.org/(7)
http://www.missgien.net/arthurian/excalibur.html(8)
http://www.timelessmyths.com/arthurian/lifearthur.html#Rise