Heavy Iron is crafted with 4 Deep Stone, 4 Obsidian and 1 Iron Ingot, as seen below.
D = Deep Stone, O = Obsidian, I = Iron Ingot
Heavy Iron tools have 1,500 Durability, about the same as Diamond. They also mine as fast as Diamond. Except, however, the Heavy Iron Sword, which does 8 damage (4 hearts) which is half a heart more than Diamond. They can mine Obsidian and Deep Stone.
Heavy Iron Armor in a full set has 24 Defence points (4 more than Diamond) and will protect you from almost anything. It has slightly less durability than Diamond. It would be very helpful against the Wither or Enderdragon, but will most likely not protect you from the dreaded Herobrine.
Heavy Iron can be used to make Iron Plating. Iron Plating is crafted in a 2x2 format, so it can be made without a crafting table. For every 4 Heavy Iron Ingots, It makes 10 Plates. 4 Plates can be combined together to make heavy plates, which are used to make Core Alloy. Heavy Iron blocks are completely explosion resistant, due to the Obsidian in it's crafting recipe. Heavy iron can also be used to craft a Reinforced Anvil.