On the surface not much. An iron alloy with more than 2.06% carbon content is called cast iron and with less it is called steel. But under the surface a lot is going on. Check the diagram above. This very foreign names are the names of iron phases and crystals that form depending on temperature and carbon content. Each of them having other physical properties and therefore having a huge impact on the part.
Also, it has a huge impact on the castability of the material. CAST iron is called so for a reason. It is easier to use in the foundry and that is why cast iron is so much cheaper than steel even if the difference is just 1 or 2% of carbon.
More information on the picture: File:Iron carbon phase diagram.svg – Wikimedia Commons
