Computer systems are any set of hardware, firmware, and peripherals, designed to work together. For personal computers, PCs, a system comprises at least the following: a system unit housing hardware and, the motherboard and any related expansion boards, disk drives (hard, floppy, CD/DVD), memory; a keyboard, a mouse and a monitor. Other devices may be present, such as a printer, modem or backup tape drive. Operating systems and application software can be thought of as parts of a full computer system, but this category focuses on HARDWARE components only. The Computers/Systems category covers hardware and software systems.