Bats are arguably the most insignificant mobs in Minecraft. They hang upside down in caves and will fly mostly at dusk and dawn. Bats, due to their erratic motion and small hitbox, are extremely hard to hit. They are a passive mob. They are one of the smallest mobs in the game.
They will almost always choose to roost in dark areas. If a player approaches them, they will "wake up", and begin flying around until they are approximately 7 blocks or more from the player, at which point they will revert to hanging upside down.
They have no mechanism of attacking or harming players, past giving players a scare with their squeaks. Upon death, they will let out a pitiful high-pitched squeal, then fall sideways following the normal pattern of mob death.