Computer peripherals are any computer components that expand system functionality and are not necessary for basic operation. Any devices that you connect to your computer, like a webcam, mouse or flash drive are all considered peripherals. However, the lines between peripheral and necessary are a little blurry within the system itself. A computer can operate with just a CPU, motherboard and power supply, making those three non-peripheral devices. Whether RAM and internal hard drives are peripherals is debatable.
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Keyboard, Mouse and Touch Pad Devices
Keyboards, mice, touch pads and other Human Interface Devices are considered peripherals. 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.
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. 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.
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.