Cast-Iron is an Iron item made in a cast.
To cast something, you basically make a mould and pour freakin hot metal into it. The metal takes the shape of the mould. Everyone knows this.
Cast-Iron actually has a lot of airspace in it (like a Mirage Bar chocolate bar) and isn't very strong. Casting is, however, a very cheap way to make something.
If you are trying to buy tools, you are better off getting stamped or forged tools. They are stronger.