The general consensus is that the grail is a pseudonym for one of the following:
1) A Grail King... a direct descendant of Jesus Christ, who would (to the medieval mindset) be heir to all Christendom.
2) The Head of John the Baptist... there is a small group of Christian heretics who live subversively alongside (traditional) Pauline Christians called Johannites who believe John the Baptist was the true Christ and Jesus was an Antichrist usurper. There are rumours that the Mandaens (swamp Kurds) of southern Iraq are actually Johannites despite their outwardly Christian/Muslim protestations. What is not known is whether any of the other historical Christian heretics: the Cathars; Bogomils; Capocratians; Knights Templar or Simon Magis disciples are actually Johannites... and whether or not any of them have survived to modern times in the form of secret societies who protect this relic.
3) The Philosophers Stone... a magical (perhaps metaphorical) stone which is claimed to make possible the Great Work (the conversion of base metals into gold). But that gets into the realm of Alchemy and hidden esoteric traditions.
4) The Ark of the Covenant... every time I go scouting locations in southern France I play the theme from Indiana Jones in my rental car.
5) The Lost Visigoth Treasure... when Alaric and the Visigoths conquered Rome in the fifth century they removed everything of value, including the entire Roman treasury (which contained the Temple treasure from Jerusalem since 70 AD) back to their capital Rhedae (believed to now be the "village of mystery" Renne le Chateau)... this treasure has never been recovered. It's probably somewhere in the region of a hundred billion dollars worth of gold, silver and artifacts.
6) The Templar Treasure... as the first bankers in Europe the Templars became immensely wealthy. When they were suppressed in 1307, King Philip le Bel seized all their assets. The Templar coin/bullion reserve was never found.
7) The Body of Jesus... in the first century the Rhedae area of southern France actually had the worlds second largest population of Jews (outside Jerusalem/Israel). If Jesus had actually survived the crucifixion and was banished/unwelcome in Israel/Judea/Palestine chances are he would have moved to southern France. Others believe Jesus and Issa are the same person and that the body of Issa/Jesus was removed/stolen by Templar Knights (or Christian Crusaders) from the mausoleum in Kashmir (between Pakistan and India). The Kashmiri tomb of a prophet/healer named Issa who supposedly claimed to be the exiled Jesus is still visited by pilgrims to this day.
The Grail Cup... the cup/vessel used in the Last Supper that supposedly also caught Christs blood when he was dying on the cross.
...there are several other theories covering everything from the ridiculous (ley-line convergence points) to the obscure (Thoth's Emerald Tablets or perhaps even copies of Berossus' Histories), but those listed above are the main contenders.
The Luke