A Red Herring is pretty simple. It's a term for a clue that leads entirely in the wrong direction, and is almost always placed there intentionally by the writer. Not to mislead the reader, necessarily, but certainly to misdirect them from the truth a little, in order to keep the suspense alive. A Red Herring is a common plot device in mysteries and crime thrillers, but is not necessarily exclusive to those genres.
The term Red Herring comes from a metaphor about hunting: the originator of the metaphor wrote about how hounds chasing after a hare might be distracted from the hunt by the smell of a red herring that had been dragged across the trail.
Mother Goose also has an unrelated, but fun rhyme which uses the term:
A man in the wilderness asked this of me,
"How many strawberries grow in the sea?"
I answered him, as I thought good,
"As many red herrings as swim in the wood."
Two of the most common examples of Red Herrings are when an innocent character is purposely cast into a guilty light by the author, in order to distract from the real perpetrator, or when a false protagonist is used. A false, or decoy, protagonist is usually carried out by clever point of view usage and or narrative distance.
Probably the best example of an innocent character cast into a guilty light is The Butler from Hound of the Baskervilles. Creeping about the mansion at night, and a beard that matches the suspect's perfectly, force the reader to wonder about him very early on.
Two well known examples of false protagonists are Lord Eddard Stark in George R.R. Martin's A Game of Thrones, and Bernard Marx, from Aldous Huxley's A Brave New World.
Just like Deus Ex Machina, it should be pointed out that a Red Herring is not necessarily a bad thing. If handled properly, it can be quite a thrilling element to storytelling. J.K. Rowling was an absolute master with this device, creating probably the most well crafted Red Herring ever written in Severus Snape.
