The Holy Grail is, and almost always has been, a MacGuffin. In fact it might be the most prolific MacGuffin in storytelling.

A MacGuffin is an item (can be an event or person, but generally just an item) that is essential to the plot of a story despite being innocuous and irrelevant in its own right. The audience doesn't care much for the item, but they do get invested in the journey. Characters from King Arthur to Indiana Jones have had many an adventure in search of the mythical piece of tableware.

The Holy Grail first appeared in Perceval: The Story of the Grail by Chrétien de Troyes in the late 12th century. In this story the grail is one of a number of magical artefacts paraded by the Fisher King in his castle. As with many depictions of the grail, here it had healing properties. And, as is often the case with MacGuffins, it's forgotten halfway through the story and was never that important. Chretien doesn't even called it The Holy Grail, it's just 'a grail'. Perceval was an unfinished work, but it was still considered an outstanding and compelling piece of literature; so compelling that there were multiple different continuations of it, with people writing new endings for decades after Chretien's death.

However, the Grail as we know it was invented by Robert de Boron, who fleshed out Chretien's grail in his Joseph d'Arimathie, written around 1200. In this story, the Holy Grail, as it was now called, was used to collect the blood of Christ at the crucifixion. That blood could then be used to heal mortal wounds and grant eternal life. Joseph of Arimathea then travels west to found a small knightly order to guard the cup.

Since Robert de Boron, some interpretations have stuck closely to this lore. Indiana Jones and the Last Crusade is clear example of this. You've got the cup, the blood of Christ, the healing properties, the order of knights. The only way it's lore differs from that of Robert de Boron is that rather than venturing west and settling in Glastonbury, the grail is kept in the Middle East. Another strand of literature, popularised by the famous English writer Thomas Malory in his story Le Morte d'Arthur, involves the grail as a symbol of divine favour and epic destiny rather than as a practical healing item. Since then there have been a wide variety of interpretations as to what the Holy Grail specifically is both physically - 'Grail' is a vague word and can be used to refer to a dish, a bowl, a cup; basically anything you might find at a table that stores a liquid - and magically.

However, a monk and historian called John of Glastonbury didn't seem to appreciate the fictionality of the grail and included it in his history of Glastonbury, composed in 1350. John didn't use the words 'Holy Grail', but did say that Joseph of Arimathea came to England with vessels containing the blood of Christ, so he may as well have done. Since then, some people have mistakenly thought the grail to be a real, historical item. But that's just the result of a 14th century chronicler not being a good historian.

So obviously, it's never been actually found. It's not a real thing - it exists because Chrétien de Troyes needed a MacGuffin for the titular character of Perceval to find and thereby have adventures.