Monkeys are primates, a group of mammals that also includes apes and lemurs. They are found in tropical and subtropical regions of Central and South America, Africa, and Asia. They have a wide range of characteristics, including:

Opposable thumbs: Monkeys have opposable thumbs, which allow them to grasp and manipulate objects. This is an important adaptation for arboreal monkeys, which spend much of their time in trees.
Long tails: Many monkeys have long tails, which they use for balance and communication. Some monkeys, such as capuchin and macaques, have prehensile tails, which can be used to grasp objects.
Fur: Monkeys have fur, which can be long and thick, or short and sparse. The fur coloration of monkeys can vary widely, from black, brown, and gray to red, orange, and yellow. Some monkeys have distinctive markings, such as black and white stripes or patches.
Size: Monkeys vary greatly in size, the smallest being the pygmy marmoset which can be as small as 14cm and weigh around 100 grams, while the mandrill can reach up to 1 meter and weigh around 35 kgs.
Teeth: Monkeys have 32 teeth, which are similar to those of other mammals. Some monkeys, such as the macaques and baboons, have large canine teeth that they use for biting and fighting.
Behavioral and cognitive abilities: Monkeys have complex communication systems, using vocalizations, facial expressions, and body language to convey information. They are intelligent and have been observed solving problems, using logic and learning from experience. They also exhibit cultural transmission of certain behaviors, such as tool use, within their troops.
Each species of monkey has unique characteristics and adaptations that allow them to survive and thrive in their specific environment.