Monkeys are Primates like Apes and Humans (we are a type of ape). Most monkeys live in Trees but some like Baboons live mostly on the ground. Old world monkeys and New world monkeys are both called, (you've guessed it) monkeys though they aren't closely related. Old world monkeys live mostly in Africa and Asia while new world monkeys live in South America. 
Are we descended from monkeys? Our closest living relative is an ape, the Chimpanzee. Monkeys and apes are descended from a common ancestor that was also our ancestor.   Apes and monkeys became separate groups 25 to 30 Million years ago. 
Some monkeys have been taught to recognise themselves in a mirror. That's something few Animals can do. Once they could recognise themselves what did they do? They had a good look at their genetals and their bottoms. They wanted to know what those parts of their bodies looked like. 
- Why Are There No Native Monkeys In North America?There are a few wild monkeys in Gibraltar in Europe but no wild monkeys North America.
- Did humans evolve from monkeys or apes?This one's an easy introduction.
- Are We Descended From Monkeys?This one's a bit technical.
- Oldest Fossils Reveal When Apes & Monkeys First Diverged
- Monkeys Learn to Recognize Themselves in a Mirror – And Promptly Check Out Their Butts