Moon

The Moon is a sprite that appears during night time.

Grass blocks or Saplings do not grow in moonlight unless there is a Torch (regular or Redstone) or other light source nearby, however they won't decay in moonlight either.

Mobs
Hostile Mobs will spawn around dark areas of a normally generated map. When the moon is up, hostile Skeleton, Creeper, Zombie and Spider mobs will appear and attack the player. When the sun comes back up Skeletons and Zombies will burn in the sunlight and drop items after death. Spiders will not burn in daylight, but instead become passive during the day, only to become hostile again if attacked or when it is night. Creepers and slimes do not burn, and will remain hostile during the day.(See also: Difficulty level.)

Trivia

 * Night time lasts 7 minutes, plus 1.5 minutes each of sunrising and sunsetting.
 * You can't see the moon while using the small or tiny render distance settings.
 * The moon is 8 x 8 pixels.