Koalas are marsupials, and monkeys are eutherians. Monkeys are primates, koalas are not. You don't need any more differences than those. read more
A monkey is a placental mammal that gives birth to fairly mature young. A koala is a marsupial that gives birth to young that are not that much more than embryos. Koalas as a marsupial have a pouch where the young go and stay until the are mature enough. Monkeys have tails, koalas do not. read more