The head of M16 leaned forward and placed an object on the top of her polished mahogany desk. It was small enough to carry but at the same time too large to easily conceal and from the satisfying clunk! it made when it came to rest it seemed to be quite heavy.
I looked at it and remembered some schematics I had seen a little while ago. "Is that a MacGuffin 2000?" I asked, trying to be one step ahead of my boss.
"No, Agent 13," she replied cooly, "This is a MacGuffin 3000. We need it delivered to a scientist in a surprisingly remote location and it is vitally important that it arrives undamaged. Do I make myself clear on this? We don't want a repeat of the Rotterdam incident."
"It will be safe with me," I assured her, "Am I taking a direct flight there and will I have any sort of backup or additional agents with me if this is so important?"
She smiled at me condescendingly, "You know the drill, Agent 13. You will be working alone despite the apparent dangers and importance of the mission and we will send you via an indirect route that will take you to many of the more scenic places on the planet where no doubt others will do their best to relieve you of both the MacGuffin and vital signs. Although I'm sure you will find plenty of opportunity to gamble in various casinos and meet beautiful women. You leave in half an hour."
"One question," I said, "What does the MacGuffin do?"
"That," she replied, "Is of no importance to you."
For anyone unaware, a MacGuffin (the term is supposed to have been coined by Alfred Hitchcock) is an object, person or idea that drives the plot forward while being itself largely unimportant. Here a MacGuffin is clearly going to be used to provide Agent 13 with an excuse to get into a series of close shaves and exciting encounters of every sort but it's just something for him to carry from A to B, it's not really going to help him in any way.
MacGuffins are very often used in films; one of my favourites is the briefcase in Ronin. So many of my friends watch the film and are disappointed that they don't get to see what's in the case at the end of the film. They are somewhat missing the point that the film isn't about the case at all, it's just an excuse for a series of car chases, shoot outs and double crossing. The same goes for Death Star plans hidden in an astromech droid or Private Ryan needing to be saved.
In literature they still exist although since books can take a little more time to develop ideas they are not always needed, or can be more subtle. There are some pretty familiar MacGuffins though. The One Ring in The Lord Of The Rings being a prime example. In the Hobbit this is simply a ring that makes people invisible but in the sequel it suddenly becomes the reason for the whole trip to Mordor. In Little Red Riding Hood granny's lunch is what gets Little Red into a dangerous close encounter with the Big Bad Wolf.
MacGuffins are useful to authors and readers; they require just enough justification for why the main protagonist should be interested in them but don't need a full explanation or even much logic behind them. But once they are established they can be usefully employed to give the plot a little push or twist.
But always look out for the Ronin problem; there the briefcase is so central to the whole film right up until the last few minutes that it's natural to wonder what is so great about it. So use a MacGuffin by all means but make sure it does not need a full explanation for the reader to be satisfied.