A monkey is any cercopithecoid (Old World monkey) or platyrrhine (New World monkey) primate. All primates that are not prosimians (lemurs and tarsiers) or apes are monkeys. The 264 known extant monkey species represent two of the three groupings of simian primates (the third group being the 21 species of apes). Monkeys are usually smaller and/or longer-tailed than apes.

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.

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.

Characteristics
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 savanna; 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 color 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.

Source: Wikipedia