The Holy Grail is the cup from which Christ drank at the Last Supper and which was used by Joseph of Arimathea to catch Christ's blood as he hung on the cross. The Holy Grail was introduced into the King Arthur legends by Robert de Boron in his romance Joseph d'Arimathie which was probably written at the very end of the twelfth century or the first decade of the thirteenth.
In earlier sources the word "grail" comes from the Latin gradale, which meant a dish brought to the table during various stages (Latin "gradus") or courses of a meal. SoChrétien de Troyes and other early writers intended this sort of platter by the term "grail." Chrétien speaks of "un graal," a grail or platter and thus not a unique holy grail.
In medieval romance, the Holy Grail was said to have been brought to Glastonbury in Britain by Joseph of Arimathea, though one would assume that to be an unlikely thing for him to have done. In te time of King Arthur the quest for the Holy Grail was the highest spiritual pursuit for a knight.
Chrétien credits Perceval is the knight who must achieve the quest for the Grail.
Malory though has Galahad as the chief Holy Grail knight, though other knights (Perceval and Bors in the Morte d'Arthur) do achieve the quest.
Tennyson is perhaps the author who has the greatest influence on the conception of the Holy Grail quest through his Idylls and his short poem "Sir Galahad".