Monkey is a common primate. A monkey is any cercopithecoid or platyrrhine primate. All primates that are not prosimians such as lemurs and tarsiers or apes are monkeys. The 264 known extant monkey species represent two of the three groupings of simian primates. 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 form part of the parvorder Catarrhini, which also includes the apes. Thus, scientifically speaking, monkeys are paraphyletic and Old World monkeys are actually more closely related to the apes than they are to the New World monkeys.
Due to its size 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.