A herring is a small teleost fish of the genus Clupea. Best-known of this family is probably the Atlantic herring. There are 15 different species of herring. When herrings migrate in the water they usually do this in large numbers; this is then called a school of herring. Like other fish, they do this for protection: see shoaling and schooling.
A red herring is used to describe a plot device in mystery fiction that leads the reader to a wrong solution.