Iron is a naturally occurring element which was first smelted from its ore into a tough silvery/white metal about 2,400 BC. Cast iron is hard and brittle, while wrought iron is soft and malleable. Iron from a blast furnace (pig iron) is an alloy of iron and carbon (about 4%), along with smaller quantities of silicon, manganese, phosphorus, sulphur and other elements.
Iron is a far less versatile metal than steel, which is also an alloy of iron. It is produced in a BOF and has a much reduced carbon content. Other elements, particularly manganese, are adjusted or added to achieve specific properties. There are many thousand different steel grades. Even high carbon steels contain no more than about 1.5% carbon, though some high alloy steels reach 2.5%. Carbon allows hardening of the steel via heat treatment. And for more detail click and see wikipedia entry.