Ist ganz einfach:

An ape refers to primates that is most closely related to humans. This includes Gorillas, Chimpanzees, Orangutans, Bonobos, and other species. They do not have tails. A monkey is a general term, and refers to many primates that have tails and live in trees.

Apes sind Menschenaffen ohne Rute (Schwanz).

Monkeys sind die kleineren Affen mit Rute (Schwanz) und leben meistens in Bäumen.