Bucks
"Buck" is a term that refers to male children who are becoming adolescents. It is similar to the terminology for classifying a male deer. "Buck" refers to a small male deer, typically less than 4 years old. Bucks have small antlers, only one tine or so, if they have any antlers at all.
Harts
Harts are usually known as red deers. They are near full maturity and live in temperate forests, woodlands and on mountains, from Europe to Asia and North America. Like most deer, their diet consists of grass, plants and twigs. They are usually between 69 to 91 inches long.
Stags
Stags are often known as red deers as well, and are considered to be full-grown in size. They come from Cervuscerus elephus, which is a European species related to elk. The male of the species has antlers called tines. A stag has a third- or fourth-generation tine, which can help determine its age, since a male deer's antlers drop off and regrow each year.
Bulls
Bulls are the largest of male deer and include the caribou that lives in the Arctic. Bulls, the oldest of male deer, have extremely large hoofs and massive antlers called "crowns" or "cups." Male deer have typically grown five sets of antlers by the time they become bulls.