There is some confusion as to how many swords Arthur possessed, magical or not.  Here is a brief listing of those listed in both historical documents and in the many legends surrounding Arthur.

The High Kings Sword:
This would have been his father Uther's sword, a symbol of his status as High King.  In several versions of the legends, this un-named weapon was brought before a convocation of all the Low Kings as Uther lay dieing.  About to acknowledge Arthur as his heir, he pulled the sword from it's scabbard and all saw that it was broken, a symbol of immense ramifications both politically and to Arthur.  What happened to this sword is unknown and it fades from the tales as other more spectacular weapons appear.  In later years, the High Kings Sword became equated with the one known as Caliburn.

The Sword in the Stone - Caliburn
Everyone who has seen the Disney version of the legend of King Arthur is at least familiar with how a young squire is dispatched to find a lost sword and reclaims one embedded in a stone, thus proving himself to be the rightful King of England.
According to some legends, the Sword was Uther's sword, the symbol of the High King of Britain which Merlin magically embedded in a stone to quell the battles among the Low Kings over who would rule in Uther's place.
Admonishing them that the one man who is able to draw the sword from the stone again will be the rightful High King, Merlin disappears once more to oversee the lessons of Arthur who is hidden with Sir Ector and his family.
Legends indicate that each year the Low Kings met to hold tourneys and attempt to pull the sword from the stone and one year Sir Ector brought his sons Kay and Arthur.  Arthur proceeds to pull the sword from the stone and returns it to Sir Ector and Cei who, briefly consider taking the throne themselves.  When Merlin reappears and declares Arthur, who was probably in his late teens, to be Uther's son proof is demanded by the knights and rulers present.  The sword is once again placed in the stone and none but Arthur can withdraw it.

Excalibur
This sword was given to Arthur by the Lady of the Lake in most versions of the legends whether she is portrayed as a high priestess of Avalon or an ancient Welsh goddess of water.  Although most legends do not mention it, Excalibur is given with a scabbard which is also magical, both items being created by magical (Elven) folk.
Excalibur is the symbol of the Protector of Britain and is unbreakable while the scabbard protects the owner from being mortally wounded or, in some versions, from bleeding.

Names for Excalibur
The mythology of the name is some what confusing for it is also said that the Welsh name for Excalibur was Caladvwlch, equating linguistically with Irish Caladbolg, the name of a sword borne by heroes in Irish legend, derived from CALAD (hard) and BOLG (lightning). It goes with a story in which the sword is struck by lighting just as the (Elven) smith takes it out of the water after its final heating.
Caladbolg is also known as the sword of the Welsh legend Cu Chullain. Caliburn is also supposed to be the old Welsh name for the sword, which was later transformed to 'Excalibur' by French poets like Chretien de Troyes. It is supposed to mean 'Cuts steal'.