Deer

Defined strictly, a deer is a ruminant mammal belonging to the family Cervidae. A number of broadly similar animals, from related families within the order Artiodactyla, are often also called deer.
Depending on the species, male deer are called stags, bucks or bulls, and females are called hinds, does or cows. Young deer are called fawns or calves. Hart is an expression for a stag, particularly a Red Deer stag past its fifth year. It is not commonly used, but an example is in Shakespeare's "Romeo and Juliet" when Tybalt refers to the brawling Montagues and Capulets as hartless hinds.
Deer are widely distributed, with representatives in all continents except Australia and Africa.
Deer differ from other ruminants in that they have antlers instead of horns. Antlers are bony growths which develop each year (usually in summer) and, in general, it is only male deer that develop them (although there are exceptions).
There are about 43 species
 of deer worldwide, divided into two broad groups: the old world group includes the subfamilies Muntiacinae and Cervinae; the new world deer the subfamilies Hydropotinae and Capreolinae. Note that the terms indicate the origin of the groups, not their modern distribution: the Water Deer, for example, is a new world species but is found only in China and Korea.
It is thought that the new world group evolved about 5 million years ago in the forests of North America and Siberia, the old world deer in Asia.

Deer are selective feeders. They have small, unspecialised stomachs by herbivore standards, and high nutrition requirememts: ingesting sufficient minerals to grow a new pair of antlers every year is a significant task. Rather than attempt to digest vast quantities of low-grade, fiberous food as, for example, sheep and cattle do, deer select easily digestable shoots, young leaves, fresh grasses, soft twigs, fruit, fungi, and lichens.
Deer have long had economic significance to humans. While they are generally not as easily domesticated as sheep, goats, pigs and even cattle, the association between people and deer is very old. Deer meat, for which they are hunted and farmed, is called venison.