How is a monkey different from other animals ?
Well, monkeys are considered to be intelligent animals
When did they come into the knowledge of humans as intelligent animals?
Catarrhini.
Simians and tarsiers emerged within haplorrhines some 60 million years ago. New World monkeys and catarrhine monkeys emerged within the simians some 35 million years ago. Old World monkeys and Hominoidea emerged within the catarrhine monkeys some 25 million years ago. Extinct basal simians such as "Aegyptopithecus" or "Parapithecus" [35-32 million years ago], eosimiidea and sometimes even the Catarrhini group are also considered monkeys by primatologists.
Lemurs, lorises, and galagos are not monkeys; instead they are strepsirrhine primates. Like
clades diverged into newer clades. It is thought the New World monkeys started as a drifted "Old World monkey" group from the old world (probably Africa) to the new world (South America).
Relationship with humans.
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 threaten agriculture) or used as service animals for the disabled.
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