Monkeys are haplorhine ("dry-nosed") primates, a group generally possessing tails and consisting of approximately 260 known living species. Many monkey species are tree-dwelling (arboreal), although there are species that live primarily on the ground, such as baboons. Most species are also active during the day (diurnal). Monkeys are generally considered to be intelligent, particularly Old World monkeys.
Lemurs, lorises, and galagos are not monkeys; instead they are strepsirrhine ("wet-nosed") primates. Like monkeys, tarsiers are haplorhine primates; however, they are also not monkeys. There are two major types of monkey: New World monkeys (platyrrhines) from South and Central America and Old World monkeys (catarrhines of the superfamily Cercopithecoidea) from Africa and Asia. Hominoid apes (gibbons, orangutans, gorillas, chimpanzees, and humans), which all lack tails, are also catarrhines but are not considered monkeys. (Tailless monkeys may be called "apes", incorrectly according to modern usage; thus the tailless Barbary macaque is sometimes called the "Barbary ape".) Because Old World monkeys are more closely related to hominoid apes than to New World monkeys, yet the term "monkey" excludes these closer relatives, monkeys are referred to as a paraphyletic group.