Steel is the most important structural material for humankind which is used in every sphere of daily life as well as in the industrialized world. Being a ubiquitous material, it gained importance over years and regarded as irreplaceable until now. For its abundance in nature, ease of fabrication, numerous engineering properties upon processing, easy availability and low cost have made it a prime material to be used for several engineering applications. For different purposes and needs, steel has been modified from past eras and thus plain carbon steel was invented over years, giving us a scope to use it for different purposes. A major portion of the daily day tools, drivers along with other applications in engineering structures, designs, and various products manufacturing are now made possible because of the invention of plain carbon steel. Here the classification of plain carbon steel has been given with proper definition along with its development history. Designation system and composition have also been discussed to know the system behind the naming of plain carbon steel in today’s world and to have a normal compositional range of it. Moreover, relevance with a phase diagram has also been given to get acquainted with the evolution of microstructures, phases, and microconstituents, at different typical compositions. At the end, description of different plain carbon steels has been shown with necessary features along with a wider range of mechanical properties and applications in order to have a comprehensive knowledge of this versatile metallic material.