A red herring refers to a device or diversion used to distract the onlooker from the original idea. Red herrings are often seen in films, adventure games, and puzzles. However, the most common use for such a device is in literature, especially mystery and thriller stories.
Simply put, a red herring is an item which has no use in the story except to distract the reader from the real culprit. It can take the form of a character, which the reader may believe to be the killer, only to discover later that he is innocent. Or it can take the form of an item which readers believe to be the clue to a discovery, but which turns out to be worthless.
The books of Agatha Christie often use a red herring to detract the reader from the actual offender. For example, in Cat Among the Pigeons, two similar crimes lead the reader to believe a particular character is a killer, but it turns out that the two murders in the book are unrelated, and so the character is actually free of blame. In The Mysterious Affair at Styles, the reader is led to believe that the two main characters hate each other, but this turns out to be a way for them to hide the fact that they conspired to kill somebody. Sherlock Holmes stories often use a red herring as a central part of the plot, and so does Edgar Allan Poe in many of his short stories.
In films, a red herring can often be found in Alfred Hitchcock stories, where characters and things turn out to be anything but what the viewer expects them to be. One of the best examples of the use of a red herring in contemporary film can be found in the movie Saw. During the whole film, two characters spend time imprisoned in a room in which a third character lies dead. Throughout the film, both characters appear to be guilty of a series of murders, until is discovered at the end that the third person in the room is not actually dead. He is, in fact, the killer.