Maras (mara) resemble hares (hare) in having large ears and eyes, a blunt muzzle, and long, slender legs. They are larger than the other cavies, weighing 9 to 16 kg (20 to 35 pounds) with a body 45 to 75 cm long and a tiny tail of up to 5 cm. Their front claws are sharp, but those on the hind feet are hooflike; the dense fur is gray with whitish underparts and has a crisp texture.

Although the rock cavy climbs trees to forage, cavies are terrestrial and colonial. They are active during the day (diurnal) or during early morning and evening (crepuscular). They den at night in burrows (burrowing) that they have excavated or that other mammals have abandoned; some species also find shelter in rock crevices, under brush piles, or in dense vegetation. Their diet is generally any available vegetation, such as grasses, forbs, leaves, and fruit. Even in cold environments, cavies do not hibernate. Breeding seasons vary among species, with gestation periods ranging from 50 to 93 days. Most cavies bear litters of 1 to 4 young, but wild guinea pigs have up to 5 young, and the domestic guinea pig may bear from 1 to 13 young per litter.

Cavies range from Venezuela to southern Patagonia but are not found in western Chile or most of the Amazon River basin. Guinea pigs occupy open grasslands, forest margins, swamps, and rocky areas. The rock cavy dwells among boulders in dry areas of eastern Brazil, whereas yellow-toothed and mountain cavies live in grassy, rocky, and brushy habitats in the Andes Mountains from Peru to Argentina at elevations of up to 4,000 metres (13,000 feet). Maras inhabit the open landscapes of semiarid grasslands, thorn scrub, and temperate steppes from Bolivia through Argentina and southern Paraguay.

Cavies represent a small segment of an evolutionary history that began in South America during the Middle Miocene Epoch (16.4 million to 11.2 million years ago). Five genera exist today, but fossils of 16 genera have been identified. Caviidae is part of the suborder Hystricognatha within the order Rodentia.