Lemurs belong to a distinct group called prosimians, which are the most primitive primates. Old World and New World monkeys belong to a different group called anthropoids.
Here's a simplified breakdown:
* Prosimians: Lemurs, lorises, galagos, tarsiers
* Anthropoids:
* Old World Monkeys: Baboons, macaques, etc.
* New World Monkeys: Capuchins, spider monkeys, etc.
So, while lemurs are primates, they are not monkeys.