Main Differences Between Monkeys and Apes
It's really easy (with rare exceptions):
Monkeys have tails
Apes do not.
Apes have "opposable" thumbs (like human hands).
Monkeys have webbing between their toes.
Also, monkeys tend to be smaller than apes, but there are exceptions to this. Monkeys tend to spend more time in trees. (Although during my time at a zoo, rare chimp escapes usually led the perpetrators straight to the tallest trees.
From here, we can get a little technical with categories. In the Ape group, there are "Greater Apes" (gorillas, chimpanzees, bonobos, orangutans) and "Lesser Apes" (siamangs and gibbons).
Groups of monkeys are broken out into "Old World Monkeys" (baboons, colobus monkeys, macaques) and "New World Monkeys" (capuchins, marmosets, tamarins).
There's a last group that include lemurs, tarsiers, and lorises. These are classified as prosimians.
In general, apes are considered to be more intelligent. They can make and utilize tools on their own and can even learn sign language. Many monkey species, on the other hand, are considered to be nuisances in their native habitats.
Following an interview I did with Dr. Jane Goodall some years ago, it was amazing to watch her interacting with the chimpanzees at our zoo. Behind the scenes, she was truly immersed in their world, making sounds and enjoying the contact, even though it was from behind bars. Primates, especially apes, are simply amazing to watch!
Photo copyright Karen Kennedy