A herring is a type of small oily fish found in the temperate, shallow waters of the North Atlantic. A smaller variant lives in the Baltic Sea.
Herring is one of various species of fishes of the genus Clupea especially the Atlantic herring (English herring) (Clupea harengus). Herrings move in vast schools (swarm), coming in spring to the shores of Europe and America, where they are caught, salted and smoked in great quantities.
Underwater video (looping) of a school of Atlantic herring Clupea harengus on its migration to their spawning grounds in the Baltic. (Video courtesy Uwe Kils)
A study conducted in 2003 at the University of British Columbia suggests that herrings might communicate by farting.  (http://www.newscientist.com/news/news.jsp?id=ns99994343)  (http://www.zoology.ubc.ca/~bwilson/herring.html). This study won the Ig Nobel Prize in 2004.
Herring have been a staple food source, especially for northern Europeans, back to 3000 B.C.E. There are numerous ways the fish is served and many regional recipes.
Herrings can be eaten raw, fermented, pickled (See Pickled herring) or cured by other techniques. A rollmop is a cured herring fillet, rolled up and preserved in vinegar which is flavoured with onions and spices.
A kipper is a split and smoked herring, and a bloater is a whole smoked herring. Both are staples of British cuisine.
A typical Dutch delicacy is raw herring (actually enzyme-cured) with raw shredded onions. To stop parasites, the herring has to be deep-frozen before the curing process.
In Sweden, Baltic herring is fermented to make surströmming. Herring soup is also a traditional soup and in Germany, North Sea herring is pickled to make Matjes, or soused herring.
Herring is also canned and expoted by many countries. A sild is an immature herring that are canned as sardines in Norway.
Figuratively, a red herring is a false lead in a mystery. In this context, red means smoked, and a smoked herring has such a strong smell that it can be used to create a false scent that causes hunting dogs to lose a trail.