Minecraft wiki entry for Boss_Update, associated text: Lots of settings changes.
Add-ons.
Commands.
Ocean monuments.
4 new blocks:
Sea lantern, Prismarine, Wet sponge, and Beacons.
6 new items:
Prismarine crystals, Prismarine shards, the Nether star, and the Potion/Splash Potion/Arrow of Decay.
3 new mobs:
Guardian, Elder Guardian, and Wither.