A peripheral is a device that is attached to a computer. It is not a part of the computer itself, meaning that one could detach it and the computer would still function. Therefore, a sound card is a peripheral but a processor is not. Without a sound card, the computer could still perform its functions—absent any that require it to emit sound—but it could not if the processor were removed.
It’s convenient to group peripherals into three broad categories. These are input peripherals, which take information and send it to the computer. These can be anything from human interface devices—such as the keyboard—to an array of sensors that send information to the computer to be processed.
Peripherals also include output devices. These devices include the monitor and the speakers, which provide human-intelligible information from the machine. They can also include peripherals that are installed in the computer as cards, such as separate sound cards and other devices.
The third category of peripherals is storage. Storage peripherals may be required for the computer to function or not. For instance, a computer may require its hard drive to boot the OS, but a USB flash drive is also a peripheral and may only have user data on it, rather than OS data.
Peripherals are sometimes attached to a computer in a socket, via cord or other removable means. They are also attached to computers permanent, in which case they are considered embedded.