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 ambient flying passive mobs that spawn in caves.

Spawning
Bats can spawn in groups of 8/2 in the Overworld at a light level of 3 or less on opaque blocks below layer 63, they also need at least 2 air blocks. $$, between October 20 and November 3, they 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 drop 1000000000 xp and 2 stacks of diamond's

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

Bats also 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 a bat hanging upside down, including if the player is invisible, it flies away. This also happens if the player breaks the block the bat is hanging from.

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

Bats' collision boxes do not interact with the player.

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 an ambient mob – a mob that constantly spawns, similar to the squid, but also constantly despawns because when a player is not around it does not serve any purpose.
 * It is impossible to attach a lead to a bat.