Bat

"The fourth smallest mob in the game ... these black rats with wings spawn constantly and produce a squeak so shrill you'll wish you were deaf. Bats are actually no physical threat to you in Minecraft, an example of a 'useless mob', but that doesn't mean we can't find them creepy. And our last pickaxe just broke, so it looks like we're trapped in this cave full of them. Save us, Batman!"

- Tom Stone

Bats are atmospheric flying passive mobs that spawn in caves.

Spawning
Bats can spawn in groups of 2 in the Overworld at a light level of 3 or less on opaque blocks below layer 63. Between October 20 and November 3, they only require a light level of 6 or less to spawn. On Superflat worlds, bats spawn below the top non-air layer, ignoring the usual layer-63 limitation.

Drops
Bats do not drop any items or experience when killed, like the villager.

Behavior
Bats are very small, being less than half a block tall and wide. They will randomly squeak. They often fly around aimlessly, tending to favor the east, and will sometimes fly into lava and catch on fire.

Bats also will often hang upside down on the underside of a solid block when idle, though they cannot hang on transparent or non-solid blocks. If a player approaches it while hanging upside down, including if the player is invisible, they start to fly away from the player. If you break the block a bat is resting on, the bat will go into the flying state and fly away from you.

Although the bat is a passive mob, they still despawn when going too far away, similarly to ocelots and squids.

Data values
Bats have entity data associated with them that contain various properties of the mob.

Trivia

 * According to Jeb, the bat is an example of atmospheric mob – a mob which constantly spawns, similar to the squid, but also constantly despawns, because when a player isn't around it doesn't serve any purpose.
 * It is impossible to attach a lead to a bat.