They are the only mammals to own wings and be able to fly actively. Scientifically they are called Chiroptera, a greek term meaning winged hand (khéir, hand, and pterón, wing). As mammals, bats have a "hand" very similar to ours. The phalanges of the fingers are elongated, and between them, as well as between the arm and the leg, a membrane has been created, called patagia, which allows it to perform an active flight. They are able not only to glide from one tree to another (such as the flying squirrel), but to flap their wings and fly for a few meters or even some hundred km.
The only finger free from the patagia is the thumb, which is equipped with a claw used by bats to climb. The patagia is a very thin membrane covered with skin, used not only for flight, but also for thermoregulation. It’s through it that bats, which don’t have sweat glands, release the excess heat produced during the flight.
Precisely because are flyer, bats are often mistaken for birds: the clearest feature that marks them is that bats are characterized by the presence of hair, while birds have feathers. In practice, these animals are not “relatives”, but simply both conquerors of the air environment.