A McGuffin (or MacGuffin) is an element, usually an object, in a movie, that serves no other purpose than to move the plot along. Alfred Hitchcock coined the term; he explained the term in an interview thusly:
It might be a Scottish name, taken from a story about two men in a train. One man says, 'What's that package up there in the baggage rack?' And the other answers, 'Oh that's a McGuffin.' The first one asks 'What's a McGuffin?' 'Well' the other man says, 'It's an apparatus for trapping lions in the Scottish Highlands.' The first man says, 'But there are no lions in the Scottish Highlands,' and the other one answers 'Well, then that's no McGuffin!' So you see, a McGuffin is nothing at all.
Quote courtesy of thequietus.com
Some of my favorite McGuffins:
The Maltese Falcon, the Maltese Falcon - The falcon was said to be important because underneath the black enamel, there are a bunch of gems. Lot of time and effort could have been saved if Sam Spade and the gang had just gotten together and robbed a bank.
Pulp Fiction, the briefcase - I've heard more enterprising fans postulate that the briefcase contained Marsellus' soul (which had been pulled out through the back of his head, hence the bandage). At any rate, we never find out what's in the thing... and it bugs me to this day.
Saving Private Ryan, Private Ryan - I'm not saying it wasn't a great movie, but the quest to save Matt Damon's character was really secondary to the recreation of the storming of the beach of Normandy. On a related note, I'm still a little queasy from watching it... twelve years ago.
And of course, there's the Holy Grail:
There have been volumes written on the topic, on what it is, what it symbolizes, and so forth, and I'm not going to add to them. But I suppose my favorite Grail story is the one in the movie The Fisher King. In it, the main characters go on a mad quest for the Grail, which turns out to be a trophy on some dude's shelf. I liked this version of the Grail because it didn't pretend to be anything other than what it was, and the filmmakers didn't use that coy The Lady-or-the-Tiger cop-out. The Grail was the journey, and that's all that was needed.
Pic is from outside the grocery store. They'll let you take pictures for a long time before they start looking at you funny.
This here is one of the stories of the grail, as told by Parry, Robin Williams' character from The Fisher King
It begins with the king as a boy, having to spend the night alone in the forest to prove his courage so he can become king. Now while he is spending the night alone he's visited by a sacred vision. Out of the fire appears the holy grail, symbol of God's divine grace. And a voice said to the boy, "You shall be keeper of the grail so that it may heal the hearts of men." But the boy was blinded by greater visions of a life filled with power and glory and beauty. And in this state of radical amazement he felt for a brief moment not like a boy, but invincible, like God, so he reached into the fire to take the grail, and the grail vanished, leaving him with his hand in the fire to be terribly wounded. Now as this boy grew older, his wound grew deeper. Until one day, life for him lost its reason. He had no faith in any man, not even himself. He couldn't love or feel loved. He was sick with experience. He began to die. One day a fool wandered into the castle and found the king alone. And being a fool, he was simple minded, he didn't see a king. He only saw a man alone and in pain. And he asked the king, "What ails you friend?" The king replied, "I'm thirsty. I need some water to cool my throat". So the fool took a cup from beside his bed, filled it with water and handed it to the king. As the king began to drink, he realized his wound was healed. He looked in his hands and there was the holy grail, that which he sought all of his life. And he turned to the fool and said with amazement, "How can you find that which my brightest and bravest could not?" And the fool replied, "I don't know. I only knew that you were thirsty."