The word deer has been applied at times to species that are not cervids, such as the musk deer (Moschus) and mouse deer (Tragulus). However, the former is now placed in a separate family (Moschidae), while mouse deer are actually primitive ruminants of the family Tragulidae. With these exclusions, Cervidae becomes the deer family, a consistent, natural grouping of species.

Deer have several other distinguishing characteristics. All deer lack the gall bladder. Females have four teats. Deer may have scent glands on their legs (metatarsal, tarsal, and pedal glands), but they do not have rectal, vulval, or preputal glands.

The requirement for nutrients and energy has severe repercussions on the ecology of deer. It confines deer to relatively productive habitats, excluding them from deserts, dry grasslands, and geologically old landscapes leached of nutrients. Moreover, it severely limits the abundance of Cervidae in mature, species-rich faunas in which many herbivore species compete for food. In order to meet their high nutrient demands, deer are specialized to exploit disturbed ecosystems. For instance, after a forest fire, an area normally passes through several ecological plant successions within a few decades before the original conditions are restored. Early plant successions normally contain an abundance of the type of plant food required by deer. Some disturbances, such as river flooding and the rise and fall of lake levels, occur annually and create local, perpetually immature, nutrient-rich ecosystems. Since disturbances such as wildfires, storm floods, avalanches, or wind-felled trees are unpredictable, deer have evolved great abilities to quickly find and colonize such transient habitats. For example, the severe ecological upheaval caused by the extreme climatic oscillations of the Ice Ages greatly favoured deer. Glaciers ground rock into highly fertile waterborne silt and wind-borne loess that refertilized landscapes and rejuvenated the soil. Extinctions swept away warm-climate competitors. From the tropics deer spread to colder and more seasonal landscapes, including the Alps and the Arctic. Like other families of large mammals that colonized extreme Ice Age environments, deer diversified and evolved into grotesque giants that had ornate coat patterns and large, bizarre antlers, which could grow only from nutrient-rich soils.

While deer tend to have broad, somewhat similar food habits, they are highly divergent in their antipredator strategies (predation). This divergence segregates species ecologically and thus minimizes potential food competition between species sharing the same space. A deer species that hides and, if discovered, departs in rapid jumps to hide again requires forests and thickets, while a highly specialized runner needs flat, unobstructed terrain to outrun predators. Specialized jumpers may choose to stay close to steep slopes and rugged terrain and thus avoid areas frequented by species that run and jump, while cliff climbers may exploit gradients and altitudes closed to others.

The family Cervidae divides into two fairly distinct groups, the Old World deer (subfamily Cervinae) and the New World deer (subfamily Capreolinae). This division reflects where the deer originally evolved; however, now it is not a geographical distinction but instead derives from their different foot structures. In the Old World deer the second and fifth hand bones (metapodia) have almost completely disappeared except for proximal, terminal remnants. In the New World deer the remnants are distal.

Old World deer with a basic four-pronged antler structure occupy temperate zones. These include the sika (C. nippon) of Japan and the fallow deer (Dama dama) of Asia Minor. The sika stands at the base of a great radiation of species that led to the red deer (C. elaphus) and elk (C. elaphus canadensis), the great cold-adapted deer of Eurasia and North America sporting five- and six-pronged antlers. The fallow deer is the last survivor of a radiation of giant Pleistocene (Pleistocene Epoch) deer, the most spectacular of which was the Irish elk (Megaloceros), which weighed 600 kg (1,300 pounds) and whose antlers spread up to 4 metres (14 feet) in width. The white-lipped deer (C. albirostris) of the Tibetan Plateau and Père David's deer (Elaphurus davidianus) of the swamps along China's major rivers complete the category of Old World deer.