Armor is used to lessen damage taken from hostile mobs and to a lesser extent, fall damage. Armor comes in four durabilities (gold, leather, iron, and diamond going from weakest to strongest) and in four types (helmet, chest plate, leggings, and boots). The type of armor determines how much it protects and the durability determines how much damage can be take before the armor deteriorates.
Helmet- 1 1/2 Armor protection
Chest Plate- 4 Armor protection
Leggings- 3 Protection
Boots- 1 1/2 Armor protection