Iron

About
Iron is a material that can be gathered through ore or loot chests which can be used to make items. (eg. Iron Armour, Block Of Iron and Steel)

Ore
Iron can be obtained through ore in the Minecraft world or through the mod

Loot Table
Iron can also be obtained through loot chests.

Mob Killing
Iron can also be obtained from killing the mobs below:

More Detail
Minecraft Wiki: Iron