Perhaps the most powerful and resourceful tool ever created by humanity is the computer. The term “computer” could mean a device that calculates. Nowadays a computer can do a variety of jobs. If you consider any field, be it engineering, healthcare, automotive devices, gaming, or entertainment, a computer is used to do the task. This chapter introduces how computers came into existence and describes the different components associated with a computing device. It starts with a definition of a computer system and explains its characteristics. It also includes a brief history of computers and the organization of various computer system components. A detailed explanation of various memory subsystems, a significant part of a computer, is also included. Finally, a brief introduction to and the functionality of an operating system are given in the last section.