Weasels are usually brown with white or yellowish underparts. In winter the coats of weasels living in cold regions turn white. Their pelts, especially that of the stoat (M. erminea), are known as ermine in the fur trade. The kolinsky (kolinski), also called the Siberian weasel (M. sibirica), is also much valued for its fur. The tail hairs are used to make artists' paintbrushes (brush).

Weasels are bold and aggressive predators (predation). They generally hunt alone, feeding principally on mice, voles, rats, and rabbits, but they also take frogs, birds, and bird eggs. Because of their narrow bodies, weasels are able to pursue and capture rodents in their burrows and to chase them through holes and crevices, under dense herbage, up trees, or into water. Although proficient at catching mice, weasels are also notorious for raiding chicken coops. Because they cannot accumulate fat and thus must eat frequently, weasels often kill more prey than they can immediately consume and will store excess food for later use. This explains the carnage often seen after they discover captive domestic fowl.

Male weasels mate with multiple females and do not provide parental care. Most species have a single litter per year, but the common, or least, weasel (M. nivalis) often has two. Sexual maturity is rapidly attained, and least weasels often breed at three months of age. Litter size varies from three to a dozen or more in some species. The young are born after a gestation period of anywhere from 35 days to more than 10 months, the latter because of delayed implantation of the fertilized egg.

The most common and widely distributed species are the stoat (called the short-tailed weasel in North America) and the least weasel. The range of both extends into polar regions. The stoat was introduced into New Zealand to control rabbits, but instead it became troublesome and now endangers many of the country's native birds. The least weasel is the smallest living carnivore; the smallest subspecies inhabits North America. It measures 11–26 cm (4–10 inches) in length and weighs only 25 grams (0.9 ounce). Larger forms of the same species occur in Russia and adjacent countries, where they are somewhat longer and considerably heavier. The range of the stoat and the least weasel overlap, and in these areas the species can be differentiated by the stoat's black-tipped tail. In North America the largest weasel is the long-tailed weasel (M. frenata); in South America it is the tropical weasel (M. africana). Both measure 25–30 cm, excluding the 10–20-cm tail; weight is 85–350 grams. With most weasels, males are usually twice the size of females.

Weasels belong to the family Mustelidae (mustelid), and there are three weasel genera in addition to Mustela. The Patagonian weasel (Lyncodon patagonicus) is a larger mustelid of the South American Pampas (Pampas, the). It is about 30–35 cm (12–14 inches) long, excluding the 6–9-cm (2.5–3.5-inch) tail. This weasel is grayish with dark brown underparts and a white stripe running across the forehead to the sides of the neck. The zorilles, or African striped polecats (two species of the genus Ictonyx), are somewhat smaller and are often found in agricultural areas. Their bodies are spotted black-and-white, and the tail, face, and back are striped. The African striped weasel (Poecilogale albinucha) is found in Africa south of the Congo Basin. Similar in habit to weasels of the genus Mustela, it is striped in light yellow and black, with black underparts and a long white tail.