Monkeys are members of the infraorder Simiiformes (the simians), and have long, powerful tails. They often carry objects and use their opposable thumbs to pick things up, such as a banana peel. They are very curious creatures and many species can be trained to perform for humans.
Most monkeys live in social groups called troops. The size of a troop depends on the species. Some, like mandrills, are very large with thick fur; others are small and have thin hair. Monkeys communicate with vocalizations, facial expressions, and body movements. They will often stare down a potential threat to prevent fights, but they also make sounds such as barks, screams, hoots, squeaks, grunts, and wails. These calls are meant to alert other monkeys of danger or signal a desire for food or a mate.
Several different types of monkey have adapted to harsh environments, including desert-like savannas and snowy mountains. For example, Japanese macaques (Macaca fuscata), known as snow monkeys, have thick fur that helps them survive the colder parts of their range in Japan. They are known to bathe in human-made hot springs to keep warm.
Most monkeys are able to climb trees well, and spend most of their time there. They are diurnal, meaning they are active during the day and sleep at night, either alone or in pairs. They are also very capable of hopping between trees, and some can even jump vertically for extended periods of time.