The document classifies and describes different types of plain carbon and alloy steels. It discusses three types of plain carbon steels based on carbon content: low carbon steels containing less than 0.25% carbon, medium carbon steels containing 0.25-0.60% carbon, and high carbon steels containing more than 0.60% carbon. It then provides details on properties, applications and heat treatment of each type. The document also classifies alloy steels into low alloy steels containing 3-4% alloying elements and high alloy steels containing over 5% alloying elements. It discusses AISI, HSLA, tool/die and stainless varieties of alloy steels.