The majority of mammals of the suborder Simiiformes, also referred to as the simians, may be referred to as "monkeys" in popular usage. However, in a broader sense based on cladistics, apes (Hominoidea) are also included, making the terms monkeys and simians synonyms in regards to their scope. Traditionally, all animals in the group now known as simians are counted as monkeys except the apes, which constitutes an incomplete paraphyletic grouping.

Some traits are common to both groups; for example, the majority of New World monkeys have prehensile tails, whereas Old World monkeys either have non-prehensile tails or no tail at all. While New World monkeys can be trichromatic, dichromatic, or monochromatic—as in the case of owl monkeys and greater galagos—Old World monkeys have trichromatic colour vision similar to that of humans. The faces of Old World and New World monkeys look quite different, despite the fact that both groups share some characteristics, such as the types of noses, cheeks, and rumps. This is true even though both groups, like the apes, have forward-facing eyes.

With the exception of lemurs, tarsiers, and lorises, any of the approximately 200 species of tailed primates are considered monkeys. Monkeys can be distinguished from apes by having a tail (even if it is only a little nub), a narrow chest, and other skeletal characteristics. Baboons and mandrills are two notable exceptions, but most monkeys have a short, flat face without a prominent muzzle. Most species can be found in tropical forests where they move about on all four limbs. All animals in tropical Central and South America, with the exception of the durukuli, are active during the day. They move around regularly in groups in search of food such as plants, birds' eggs, smaller animals, and insects. As a result of their ability to sit up straight

Monkeys are renowned for their intellect and curiosity, and they have enormous brains. They have a wide range of activities available to them thanks to their developing brains, freed up hands, and sharp vision. Most are adept at working out complicated problems and picking up new skills through experience, but they still fall short of great apes in terms of cognitive ability. Some people, particularly capuchin monkeys (genus Cebus), willfully use things as tools (e.g., stones to crack nuts). Others, like baboons, are easily taught how to utilise sticks to catch food. However, most monkeys don't seem to be very adept at learning from each other's experience—individuals more or less have to acquire new behaviours for themselves—in stark contrast to the great apes (gorillas, chimpanzees, and orangutans). The Japanese are a notable exception.