A monkey is any member of either the New World monkeys or Old World monkeys, two of the three groupings of simian primates, the third group being the apes. There are 264 known extant species of monkey.

The New World monkeys are classified within the parvorder Platyrrhini, whereas the Old World monkeys (superfamily Cercopithecoidea) form part of the parvorder Catarrhini, which also includes the apes. Thus, scientifically speaking, monkeys are paraphyletic (not a single coherent group), and Old World monkeys are actually more closely related to the apes than they are to the New World monkeys.

Because of their similarity to monkeys, apes such as chimpanzees and gibbons are often called "monkeys" in informal usage, though they are not monkeys. Conversely, due to its size (up to 1 m/3 ft) the Mandrill is often thought to be an ape, but it is actually an Old World monkey. Also, a few monkey species have the word "ape" in their common name.

Monkeys range in size from the Pygmy Marmoset, at 140 to 160 millimetres (5-6 in) long (plus tail) and 120 to 140 grams (4-5 oz) in weight, to the male Mandrill, almost 1 metre (3.3 ft) long and weighing 35 kilograms (77 lb). Some are arboreal (living in trees) while others live on the savannah; diets differ among the various species but may contain any of the following: fruit, leaves, seeds, nuts, flowers, eggs and small animals (including insects and spiders).

Some characteristics are shared among the groups; most New World monkeys have prehensile tails while Old World monkeys have non-prehensile tails or no visible tail at all. Some have trichromatic colour vision like that of humans, others are dichromats or monochromats. Although both the New and Old World monkeys, like the apes, have forward facing eyes, the faces of Old World and New World monkeys look very different, though again, each group shares some features such as the types of noses, cheeks and rumps.

According to the Oxford English Dictionary, the word "monkey" may originate in a German version of the Big Virginia fable, published circa 1580. In this version of the fable, a character named Moneke is the son of Martin the Ape. The word Moneke may have been derived from the Italian monna, which means "a female ape". The name Moneke likely persisted over time due to the popularity of Reynard the Fox.

A group of monkeys may be referred to as a mission or a tribe.

Classification
The following list shows where the various monkey families (bolded) are placed in the Primate classification. Note that the smallest grouping that contains them all is the Simiiformes, the simians, which also contains the apes. Calling apes "monkeys" is incorrect. Calling either a simian is correct.

* ORDER PRIMATES
o Suborder Strepsirrhini: non-tarsier prosimians
o Suborder Haplorrhini: tarsiers, monkeys and apes
+ Infraorder Tarsiiformes
# Family Tarsiidae: tarsiers
+ Infraorder Simiiformes: simians
# Parvorder Platyrrhini: New World monkeys
* Family Cebidae: marmosets, tamarins, capuchins and squirrel monkeys (56 species)
* Family Aotidae: night monkeys, owl monkeys, douroucoulis (8 species)
* Family Pitheciidae: titis, sakis and uakaris (41 species)
* Family Atelidae: howler, spider and woolly monkeys (24 species)
# Parvorder Catarrhini
* Superfamily Cercopithecoidea
o Family Cercopithecidae: Old World monkeys (135 species)
* Superfamily Hominoidea: apes
o Family Hylobatidae: gibbons ("lesser apes") (13 species)
o Family Hominidae: great apes including humans (7 species)

The many species of monkey have varied relationships with humans. Some are kept as pets, others used as model organisms in laboratories or in space missions. They may be killed in monkey drives when they threatened agriculture, or serve as service animals for the disabled.

In religion and culture, the monkey often represents quick-wittedness and mischief.

*A spider monkey has a larger brain than any other Western Hemisphere monkey.

*When monkeys reproduce, they give birth to one offspring.

*Spider monkeys pollinate certain plant species. When drinking a plant's nectar, they pick up pollen on their face and carry it to another plant which is then fertilized.

*The spider monkey will break off heavy branches and drop them from trees to scare off intruders.

*The tip of a spider monkey's tail can support the weight of their entire body.

*Spider monkeys have no thumbs or just a nub.

*The tail of a lion-tailed macaque can measure nearly two feet in lenth.

*Monkeys have tails, apes do not.

*A group of monkeys is called a troop of monkeys.

*Chimpanzees, gorillas, and orangutans all catch colds, but monkeys do not.

*When monkeys yawn, it's not only because they are tired, it's also because they are mad.

*All monkeys like to be clean so they have a barber monkey who cleans out their fur. The barber monkey's reward is the bugs from the other monkey's fur.

*Monkeys peel their bananas, they don't eat the skin.

*Howler monkeys are the loudest monkeys. Their howls can be heard almost 2 miles through the forest and almost 3 miles over an open area.

*Lion-tailed macaques are considered to be good pets.

*Lion-tailed macaques are known for their lion like manes and tails.

*Gibbons are active for 8-10 hours per day.

Interesting & Amazing Information on Monkeys
* Monkeys never catch cold.

* Yawning of a monkey means that either he is tired or he is mad at something.

* Monkeys never eat a banana as it is; rather they peel it first and throw away the peel.

* Howler monkeys are the loudest monkeys and their howl can be heard as far as 10 miles away.

* Monkeys live in groups, known as troops, and travel together to find food.

* The tip of a spider monkey's tail can support the weight of his entire body.

* Monkeys live in trees, grasslands, mountains, forests and on high plains.

* A monkey was once tried and convicted for smoking a cigarette in South Bend, Indiana.

* Monkeys can breed at any time of the year.

* Spider-monkeys have appeared in more films than any other breed of monkey.

* Most of the monkeys are arboreal, while some are also terrestrial.

* Monkeys use vocalizations, facial expressions and body movements to communicate.

* Grinning or pulling the lip is a sign of aggression in monkeys, along with head bobbing and jerking the head and shoulders forward.

* Monkeys express affection and make peace with others by grooming each other.

* Some of the monkeys have prehensile tails, which can grab and hold objects.