The proportion of carbon greatly influences the properties of the steel, but it is usually supplemented by other elements – most frequently manganese and silicon – to achieve certain physical properties such as hardness, ductility and strength.
Carbon steel is the most common form. Within this grouping mild steel generally has a carbon content of up to 0.25%, medium carbon steel contains carbon in the range 0.25-0.45%, and high carbon steel is above 0.45%.
Alloy steels, for example SBQ grades, and stainless and tool steels, have additional elements added, such as tungsten, chromium and nickel, to achieve a very specific in-service performance. And for more detail click and see wikipedia entry.