A seemingly important but ultimately trivial plot element that exists purely to facilitate the telling of the story. The term was coined by Alfred Hitchcock, who employed McGuffins with relish.
McGuffins are commonly found in in genre movies; thrillers, adventures, comedies and such. The characters must arrive at position A or problem B, and the McGuffin is there to see that they do. McGuffins are plentiful and easy to spot, but here are some examples:
McGuffins are a regular feature on The Simpsons, where it's not uncommon for the first act to be devoted to an elaborate setup for the main plotline (e.g. the soccer riot that leads to Homer buying a gun). The use of a McGuffin is often acknowledged in the show with a self-deprecating comment; Hitchcock, too, was not above drawing the audience's attention towards irrelevant plot elements.
The McGuffin can be thought of as a variable in the plot: Indiana Jones faced a great deal of peril in his quest for the $mcguffin. Lost Ark? Temple of Doom? In true McGuffin style, it's not where the audience is going, but how they get there.