A red herring by definition is a person that gives your readers a false trail to follow. They fall under the "whodunnit" category and can lead the reader to falsely believe someone else committed the crime while concealing the identity of the true villain. You cannot have a mystery without them.
When I was growing up I loved the game CLUE. And it serves as a good example of what a red herring is. Lots of suspects who all look guilty in some way. There was nothing as satisfying as getting the right person, the right weapon, in the right room. Was it Miss Scarlet in the library with a knife, or could it have been Prefessor Plum in the billiard room with a revolver?
As your reader tries to put the pieces of the puzzle together, you want them asking themselves who wanted the person murdered and why. You don't want the reader to be able to guess the villain. If your story clearly points to one possible suspect, you will be in jeapardy of your book being put down without being read all the way through.
Sprinkle in a few red herrings with all of your suspects having a possible motive to kill the victim. Make them suspicious, and have them reveal things about themselves through the protagonist - maybe by way of interrogation, catching them in a lie, etc.
I have already covered the need for red herrings, but I also want to point out that your don't want a zillion of them. 3-5 max would be okay but if you get too many in there your reader might get confused, and you don't want the book going in 100 different directions.
Want more information? Check these out:
Planting Clues and Red Herrings
How to use Clues and Red Herrings
Writing Fiction Right
For more information, visit my blog at: http://unearththeclues.blogspot.com/