The so-called typical tree squirrels are members of the genus Sciurus, with about 40 species distributed throughout forested regions of Eurasia and the Americas. These are day-active animals with slender bodies, sleek, thick fur, and bushy tails. Their coats are black, gray, brown, or reddish above and light-colored below. Light, swift, and agile, tree squirrels leap from branch to branch and scurry up and down trees using their sharp claws to dig into the trunk; they always descend head first. The tail is used as a rudder when the animal leaps and as a parachute when it drops. They have excellent sight, including good color vision. The handlike forepaws are used for holding food. Tree squirrels make nests in holes in trees or on branches. They spend much time on the ground, foraging for fruit, nuts, and insects; they also sometimes eat eggs, young birds, and smaller mammals. Members of many species store food for the winter in holes or buried in the ground; they locate these stores by means of smell. They do not hibernate.

Sciurus species include the Eurasian red squirrel, S. vulgaris, and the North American gray squirrels, fox squirrel, and tufted-eared squirrels. Gray squirrels have tails about as long as the combined head and body length. The eastern gray squirrel, S. carolinensis, common in the eastern half of the United States and extreme southern Canada, is up to 20 in. (51 cm) in total length, 5 in. (13 cm) high at the shoulder, and weighs 1 to 1 1/2 lb (450–700 grams). It has been introduced in Europe. The western gray squirrel, S. griseus, of the U.S. West Coast, is slightly larger. The fox squirrel, S. niger, is the largest North American squirrel, reaching 29 in. (74 cm) in total length; its head is somewhat square. It displays great variation in its fur color but is commonly light brown. It is found in the eastern half of the United States, excluding the extreme northeast. Although its numbers have been greatly diminished by hunting and clearing, it is still common in some areas. It has also been introduced in city parks in western states. The tufted-eared squirrels, also called tassel-eared, or Abert, squirrels, are very distinctive, with tall plumes of hair on their ears. They inhabit yellow pine forests of the Colorado Plateau. One variety, the Kaibab squirrel, is found only on the northern rim of the Grand Canyon. North American red squirrels, also known as pine squirrels and chickarees, are species of the genus Tamiasciurus. They are small and noisy, about 12 in. (30 cm) long and 3 1/2 in. (9 cm) high, weighing 5 to 10 oz (140–280 grams). They are found in the pine forests of Alaska, Canada, and the N and W United States. Other genera of arboreal squirrels are found mostly in Africa, S and SE Asia, and Central and South America.

Squirrels are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia, family Sciuridae.