lem·ming   [lem-ing]
any of various small, mouselike rodents of the genera Lemmus, Myopus, and Dicrostonyx, of far northern regions, as L. lemmus, of Norway, Sweden, etc., noted for periodic mass migrations that sometimes result in mass drownings. (Source: dictionary.com)

Lemmings weigh from 30 to 112 g (1.1 to 4.0 oz) and are about 7 to 15 cm (2.8 to 5.9 in) long. They generally have long, soft fur, and very short tails. They are Herbivory, feeding mostly on leaves and shoots, Poaceae, and Cyperaceae in particular, but also on roots and bulbs. At times, they will eat grubs and larva. Like other rodents, their Incisor grow continuously, allowing them to exist on much tougher forage than would normally be possible.

Lemmings do not Hibernation through the harsh northern winter. They remain active, finding food by burrowing through the snow and utilizing grasses clipped and stored in advance. 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 when food is plentiful. (Source: wikipedia.com)

Lemming populations shrink and swell depending on how many plants and berries are available. One type of lemming, the Scandinavian lemming, migrates in a huge group when food becomes scarce. When such a migration occurs, some lemmings die by falling over cliffs or drowning in lakes or rivers. These deaths are not deliberate “suicide” attempts, however, but accidental deaths resulting from the lemmings’ venturing into unfamiliar territories and being crowded and pushed over dangerous ledges. In fact, when the competition for food, space, or mates becomes too intense, lemmings are much more likely to kill each other than to kill themselves.

“The Norton Sound Eskimo have an odd superstition that the White Lemming lives in the land  beyond the stars and that it sometimes comes down to the earth, descending in a spiral course during snow-storms.  I have known old men to insist that they had seen them coming down.  Mr. Murdoch records this same belief as existing among the Point Barrow Eskimo.” (Source: Arctic Studies Center)

The misconception of lemming “mass suicide” is long-standing and has been popularized by a number of factors. In 1955, Disney Studio illustrator Carl Barks drew an Uncle Scrooge adventure comic with the title “The Lemming with the Locket”. This comic, which was inspired by a 1954 American Mercur article, showed massive numbers of lemmings jumping over Norwegian cliffs.
READ HERE: The Lemming with the Locket

Even more influential was the 1958 Disney film White Wilderness, which won an Academy Award for Documentary Feature, in which staged footage was shown with lemmings jumping into certain death after faked scenes of mass migration.

A Canadian Broadcasting Corporation documentary, Cruel Camera, found that the lemmings used for White Wilderness were flown from Hudson Bay to Calgary, Alberta, Canada, where they did not jump off the cliff, but were in fact launched off the cliff using a turntable.

Starts at 02:31 about White Wilderness and 3:27 about the Lemmings specifically.

This same act was also used in the Apple Computer 1985 Super Bowl commercial “Lemming”

Another commercial using the misconception of the Lemmings.

and the popular 1991 video game Lemmings, in which the player must stop the lemmings from mindlessly marching over cliffs or into traps.