The affairs of smallish, non-human primates - both New World and Old.
This term may also refer to the activities of non-monkeys when said activities resemble the activities of true monkeys.
While I believe monkeys are a truly noble breed and may one day wrest stewardship of the planet from our dirty hands, many view the monkey as a goofy, irresponsible prankster, and thus the term "monkey business" connotes mischievous, playful activity.
See Also: shenanigans.