Computer peripherals are the devices you use to expand your system's functionality and are not essential for the computer to work. Many peripherals are external devices that give you a way to input information. For example, you might use a mouse or trackpad to navigate the screen, a keyboard to type text or a microphone to record audio. Other peripherals are output devices that let you see, print or listen to something, such monitors, printers and speakers. While debatable since they are key components for a functional computer, internal devices like hard drives and memory may be considered peripherals.
Keyboard, Mouse and Touch Pad Devices
Keyboards, mice, touch pads and other Human Interface Devices are very common peripherals of computer systems. A HID is a peripheral that lets the computer use input data or interact with the computer. HIDs are always considered peripheral devices because it is possible for a computer to operate and do countless jobs without the need for human interaction. You can disconnect an HID from a computer and replace it with a new one without affecting the core functionality of the system.
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Data Storage Devices
Data storage devices are hardware devices capable of holding information and include system RAM, internal hard drives, external hard drives, solid state drives, flash drives, memory cards and cassettes. Any type of storage device that connects outside of the computer, or isn't necessary for the computer to run, is considered a peripheral. For example, you can connect and disconnect external hard drives, flash drives and memory cards without disabling the the computer. However, system RAM and internal hard drives straddle the peripheral line. While the motherboard and CPU have small amounts of built-in memory, alone, they are not enough to utilize most of the computer's capabilities.
Audio and Video Output Devices
Unless a computer operates in a self-contained situation, without any need for human interaction, it's necessary for the computer to convey information to the user. The computer uses peripheral devices like monitors, speakers and screen readers to translate the system's binary data stream into something people can understand and interact with. Computer peripheral devices encompass all devices that the system uses to output data, including printers. The data output devices aren't essential for the computer to operate, they're just needed to make the system useful.
Computer Peripheral Cards and Components
Any type of peripheral card, like a graphics card, sound card, network card or controller card, is considered a peripheral. Most modern non-storage based internal storage devices use the Peripheral Component Internet and PCI Express connection standards. The connection standard name itself implies that what ever hardware uses it is a peripheral device. While peripheral devices are necessary for things like displaying video output and communicating with other computers on a network, the computer can still operate without them.