Minecraft wiki entry for How_to_play, associated text: 'Peaceful — in Peaceful, no hostile mobs spawn except for shulkers and the Ender Dragon, but shulkers don't do any damage to you. Vindicators and evokers also spawn inside woodland mansions, and they also ignore you. The Ender Dragon still does damage and can hit you high into the air.
Easy — in Easy, hostile mobs do less damage, you aren't affected by certain status effects, and if your hunger level drops to zero, you don't lose health.
Normal — in Normal, hostile mobs deal normal damage, and if your hunger level drops to zero, your health begins to drop, but you don't die.
Hard — in Hard, hostile mobs do more damage, zombies break through doors, and if your hunger level drops to zero, your health also drops and you could die.