Both cast iron and steel are alloys of iron that include carbon; however, steel contains less carbon. According to Tata Steel Europe, steel is defined as an iron alloy that has a carbon content of less than 1%. Modern cast iron has a carbon content of between 2.4% and 4%, but historically that has varied widely due to technological difficulties with keeping the alloy mixture consistent.
Cast iron is simpler and quicker to make than steel; it is also much more brittle. It is harder for a blacksmith to work into useful shapes because it is easy to break by hammering. The "cast" part of cast iron's name refers to this fact: it can most readily be formed into useful shapes by casting it in a mold rather than by working it after it has been removed from the forge.
Steel, on the other hand, is easier to work by hammering. Steel can also be hardened by heat rather than melting as cast iron does. It has greater tensile and compressive strength as well, making it superior to cast iron for bearing weight, as in making support columns for a building. Steel's carbon content is controlled by the addition of other elements, manganese and silicon. Steel is created by melting pig iron, a crude kind of cast iron, and removing some of the carbon content by adding these elements to the alloy mix.