Together with the voles and muskrats, they make up the subfamily Arvicolinae (also known as Microtinae), which forms part of the largest mammal radiation by far, the superfamily Muroidea, which also includes the rats, mice, hamsters, and gerbils.

Lemmings mostly weigh between 30 g and 112 g (1 to 4 oz) and are about 7 cm - 15 cm (2.75 to 6 in) long. They usually have long, soft fur and very short tails. They are herbivorous, feeding mostly on leaves and shoots, grasses, and sedges in particular, but also roots and bulbs in some cases. Like many rodents, their incisors grow continuously, allowing them to exist on much tougher forage than would otherwise be possible.

Lemmings do not hibernate through the harsh northern winter, but remain active, finding food by burrowing through the snow, and utilising grasses clipped and stored in anticipation. They are solitary animals by nature, meeting only to mate and then going their separate ways, but like all rodents they have a high reproductive rate and can breed rapidly in good seasons.

There is little to distinguish a lemming from a vole. Most lemmings are members of the tribe Lemmini (one of the three tribes that make up the subfamily)

Population fluctuations

Lemming populations go through rapid growths and subsequent crashes that have achieved an almost legendary status, largely because of the well-known Disney Studios film, White Wilderness, which was produced in 1958 and reappeared on television at regular intervals for many years afterwards. White Wilderness popularized, using staged footage, the myth that during population booms Norway Lemmings become suicidal and leap en masse off cliffs into the sea. For this reason, the term "lemming" is often used in slang to denote those who mindlessly follow the crowd, even if destruction is the result.

In fact, the behavior of lemmings is much the same as that of many other rodents which have periodic population booms and then disperse in all directions, seeking the food and shelter that their natural habitat cannot provide. (The Australian Long-haired Rat is one example.)

Myths about lemmings go back many centuries, and in the 16th and 17th centuries there was much speculation in learned circles that lemmings were in fact spontaneously generated by conditions of the air. This was argued against, successfully, by the natural historian Ole Worm, who provided one of the first published dissections of a lemming. In his investigation, Worm showed that a lemming contained anatomy similar to most other rodents, including testes—a highly pointless organ were they really to reproduce, literally, out of thin air.

The populations of predatory creatures like foxes and owls follow the population changes of lemmings and voles.