There are more than 260 known species in the world, split into two groups: new world and old world monkeys. Some examples of new world monkeys are spider monkeys, tamarins, howler monkeys and marmosets, while the old world group includes macaques, baboons, mandrills and velvet monkeys.
Old world monkeys are those found in Asia and Africa, while new world monkeys live in Central and South America. There are a few noticeable differences between the two groups, as most old world monkeys have opposable thumbs whereas new world monkeys do not. In addition, many new world monkeys have prehensile tails that can be used to grab on to things, while old world monkeys can either do little to nothing with their tails or have no tail at all.
In addition to monkeys, the primate family also includes apes and gibbons, which are related to monkeys but in a separate class of their own. The ape family includes gorillas, chimpanzees and orangutans. There are 13 known species of gibbons, all of which are more similar to monkeys than the other apes, which is why they are sometimes known as lesser apes (as opposed to great apes).
Baboons are one species that there is some debate about, as they were classified as apes for many years. However, more recently scientists have determined that they belong to the monkey family for a number of different reasons.