Bat is a generic term that refers to mammals that fly under their own power using wings. This separates them out from "flying" squirrels and sugar gliders. While there are just 43 species of gliders, there are over 1000+ species of bats. Most people never even notice the bats even though they may be out in high numbers, this is because bats are nocturnal and fly at night. Unlike their avian counterpart, owls, bats in general have very poor vision, thus the expression "blind as a bat". So how do bats manage to navigate in the dark!
Bats have extremely poor eyesight, but exceptionally good hearing and a very good set of lungs. They emit sounds and hear in the ultrasonic range. They produce several hundreds of those sounds a second and then the sounds bounce back when they hit something, the bat actually hears the bounce and, what is truly amazing, can tell where and what the object is. They can also remember the bounces over time and thus find their way home. This is referred to as echolocation. Especially amazing is that each bat recognizes only its own voice, and can totally ignore all others.
Inside a cave or tunnel, there can be over 1 million bats, and they will all leave in a matter of a few minutes after the sunsets, all shouting at the top of their lungs and each one knowing where it is in relationship to all others. Imagine placing just 100,000 people in a football stadium on a cloudy night with no lights and everybody blindfolded, screaming at the top of their voice and then being able to find the exits without running into anybody else. Bats do this all the time.
Not only can they navigate by bouncing the signals off trees, rocks and buildings, but they can navigate to find moving objects as well! Echolocation is how small insectivore bats locate mosquitoes, and one bat can find and eat thousands of bugs a night. There are even bats in the Caribbean that find small fish under the surface of the water using echolocation! The bats vocalisations and hearing can navigate them just about anywhere and through anything!
Echolocation does have one small problem that most never think about, bats can't navigate in the rain! All the raindrops bounce back their signals and they just have to stop and sit. This explains how bats sometimes get stranded in trees or bushes around golf courses, the sprinklers were turned on and they had to stop flying. At least it gives their vocal cords a break!