The American Iron and Steel Institute categorizes steel into four main categories based on chemical composition: carbon steel, alloy steel, stainless steel and tool steel. There are also many different grades of steel, although all types are made of iron and carbon.
Types of steel are also classified by a variety of other factors, such as production method, finishing method, form and microstructure. Alloy steel is a type of steel with one or more alloying elements added to it. Stainless steel is modified to increase resistance to corrosion than standard steel. Tool steel is a class of highly-resistant, extra hard steel. Carbon steel is separated into three main categories: mild, medium and high carbon steel.