Steel is an alloy consisting of iron and carbon. Steel is a term used mainly to refer to iron alloys with low carbon content (typically less than 1.9%) or containing a low level of additions like chromium, such that they can be reshaped when in a heated state. Steel differs in this respect from cast iron, for example, which has higher carbon content.
Many different alloys contain these two elements, usually with other constituents, and there are therefore many kinds of steel. Because of the broad range of choice and favourable processing characteristics, it is a widely used and durable construction material. The carbon is used to achieve a high level of tensile strength and hardness.