Moon

The Moon appears during night time. It is the harbinger of darkness.

Grass blocks or Saplings do not grow in moonlight unless there is a torch (regular or Redstone) or other light source nearby.

Mobs
Mobs (monsters) will spawn around dark areas of a normally generated map. When the moon is up, hostile skeleton, creeper, slime, zombie and spider mobs will appear and attack the player. When the sun comes back up Skeletons and Zombie mobs will burn in the sunlight and drop items on death. Spider mobs will still be there but are peaceful mobs during the day. Creepers DO NOT go away and it is unknown what happens to slimes because they are so rare. (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 if you have the small or tiny render distances.
 * The moon is 8 x 8 blocks.