A peripheral is a device that is connected to but not an integral part of a host computer. It expands the host's capabilities without being part of the core architecture. There are three types of peripherals: input devices like keyboards and mice that send data to the computer, output devices like monitors and printers that provide output to users, and storage devices like hard drives that store processed data. Output devices take electronic signals from computers and convert them into human-readable forms by methods like displaying on a monitor or printing. Common output devices include monitors, printers, LCD/projectors, and speakers.