What is a Network Interface Card (NIC) and why might a device have multiple NICs?

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Accepted Answer

A NIC is the hardware that physically connects a device to a network; a device may have several (e.g. one wired, one wireless) to attach to different networks or media.
Before any network communication can happen at all, a device needs a physical connection to a local network — and the Network Interface Card (NIC) is the piece of hardware that provides it. That connection can be wired (copper or fiber) or wireless (radio), depending on how the network is set up, and the same logic applies whether you are in a corporate office or at home.
A device can carry more than one NIC. A laptop, for example, typically has both a wired Ethernet NIC and a wireless NIC, letting it attach to different networks or media and switch between them. This matters because not all physical connections offer the same performance: a wired link is usually faster and more reliable than a wireless one, so the choice of NIC directly affects how well the device communicates.
Go deeper:

What is a network interface card (NIC)? — TechTarget — concise definition of the NIC as the hardware that connects a device to a network.

Network Interface Card (NIC) — GeeksforGeeks — contrasts wired (Ethernet) and wireless NICs, which is exactly why a device carries more than one.

Network interface controller — Wikipedia — broader reference on NIC hardware and how it implements the physical layer.