The higher the carbon percentage of the carbon steel, the harder and stronger the steel can become through heat treating. However, as the carbon percentage rises, the carbon steel also becomes less ductile. Higher carbon percentages also reduce weld-ability and lower melting temperature.
Low carbon steel is hard but not brittle, and typically used for things like chain link fences, railings and gates. It is hard enough to serve its purpose, but flexible enough to be worked easily.
Steel with medium carbon content is the most commonly used in construction and for structural purposes. You will find it in the building of railways and bridges, large automotive parts and ships, as well as home appliances such as washing machines and refrigerators.
High carbon steel is the hardest, but also most brittle. It is most often used in manufacturing where strength is the most desirable attribute. Ideal for high tension wires and springs, as well as industrial components such as knives, punches, dies and other cutting tools.