A monkey is any cercopithecoid (Old World monkey) or platyrrhine (New World monkey) primate. All primates that are not prosimians or apes are monkeys. The 264 known extant monkey species represent two of the three groupings of simian primates (the third group being the 22 species of apes). Monkeys are generally considered to be intelligent and, unlike apes, monkeys usually have tails.
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.