A
plot device is a person or an object introduced to a
story to affect or advance the
plot of the story. In the hands of a skilled writer, the reader or viewer won't notice that the device is a construction of the author -- it will seem to follow naturally from the setting or characters in the story. A poorly written story, on the other hand, may have such awkward or contrived plot devices that the reader has serious trouble maintaining
suspension of disbelief; indeed, they may even leave
plot holes.
A MacGuffin is a plot device which is unimportant in and of itself, but is the catalyst for the action which surrounds it.
See also deus ex machina; quest.