This is a term coined by the great film director Alfred Hitchcock in 1935. The formal definition is: an object, event, or character in a film or story that serves to set and keep the plot in motion despite usually lacking intrinsic importance.
So, my question is: What is the difference in a MacGuffin and a Red Herring? I found a very good explanation here:
A MacGuffin is a person, incident or object that motivates the other characters. It is the thing everyone is searching for or talking about-the stolen jewelry, the mysterious contents of a case in Pulp Fiction and Ronin, the money Janet Leigh’s character steals in Psycho, Jack Sparrow’s compass in Pirates of the Caribbean: Dead Man’s Chest. The real story, however, lies in what the MacGuffin motivates the characters to do, it doesn’t matter, in the end, what the MacGuffin is.
Unlike MacGuffins, Red Herrings are meant to distract the audience and lead them in the wrong direction, diverting attention to the incorrect conclusion in a mystery or a crime drama. They are a sleight of hand that distracts from the real story, the right solution to the crime or a problem. The army of the Twelve Monkeys in Twelve Monkeys is a good example, as is Severus Snape in Harry Potter and the Goblet of Fire. Many critics feel that red herrings are overused and are becoming cliched attempts to impose plot twists on poorly written stories, but Red Herrings remain a necessary element of telling a story.
The main difference between a Chekhov’s Gun element and a MacGuffin is that the Chekhov’s Gun seems insignificant at first, and we later learn that it is very important; whereas a MacGuffin or Red Herring seems incredibly important, but we later learn that it is not.
Don't you just love all of this?!