A herd is a large group of animals. In birds, it is called a flock (and with geese, sometimes a gaggle).
Why do animals herd?
Herding is mostly an instinctive defensive tool of preys against predators, that hesitate to attack isolated animals, but some predators find also strength advantages in acting together.
The structure of herds
Domestic herds
Domestic animal herds are assembled by humans for practicality in raising them and controlling them.
Human parallel
The term herd is also applied metaphorically to human beings in social psychology, with the concept of herd behaviour. It seems that herding brings them a feeling of strength or safety.
Among other applications, it is used in behavioral finance to describe the largest group of market investors or market speculators who tend to 'move with the market,' or 'follow the general market trend.'
Market trends move in bull markets and bear markets, among many other periods of mixed or indeterminant markets. This is attributed by some researchers to rational decisions (information cascade , rational expectations...) and by others to pure emotional mimicry, fear and greed contagion.
"Contrarians" or contrarian investors tend to invest or speculate counter to the herd.
See also