Carbon steel is a common choice for structural and other uses. In fact, it's by far the most commonly produced kind of steel in the United States. In addition to being high in strength and ductility (its ability to bend into different shapes without breaking), carbon steel is relatively easy to make. It's an alloy, which is one metal that's mixed with other metals or non-metallic elements. As the name would indicate, carbon steel is mainly composed of iron and carbon, with a few other elements often alloyed with it in very small amounts. This kind of steel is made in a blast furnace; iron and carbon from coke (an industrial fuel made from coal or oil) mix together as the iron become molten. After going through a few other processes like deoxidation, the steel is cast into a mold. Afterwards, it can go through a number of processes like rolling, cold forming, and hot forming to create the exact type of steel you need for your project.
Because of how common it is, carbon steel can be used in a huge variety of applications. From large buildings to bolts and fasteners, you'll find carbon steel in a number of places. However, there are other types of steel that can be useful for more specific purposes.