Prehensile tails evolved twice in New World monkeys, and the version found in spider monkeys, woolly monkeys and howlers includes a naked ventral pad. That pad carries dermatoglyphs, the ridged friction skin used elsewhere for grip, along with a dense supply of touch receptors and its own large representation in the somatosensory cortex.
The tail functions as a fifth limb. It supports the whole body during suspensory feeding, which lets the animal reach fruit at the ends of branches too thin to walk on, and it can manipulate objects. In parallel, spider monkey hands have lost the external thumb, leaving four long curved fingers that work as a hook for brachiation.
The tail is often called a fifth hand, which flatters it. It has no opposable digit and cannot perform precision grips; it clamps and curls. The comparison holds for weight-bearing and sensitivity, not for dexterity.