Network Interface Controller (NIC) or Network Card is a device that enables communication between devices of a particular computer network. Computers can be connected either with a cable or using wireless.
How does it work?
The network card provides connection between the Computer and the outside world. Every device (e.g. desktop, notebook, tablet, phone, or TV that communicates over a computer network) can be equipped with a network card. Essentially, this card represents a given device on a network where it can be unambiguously identified thanks to MAC address which thus allows to track back a device along with its user. Therefore, MAC address can be considered personal data. If the device has multiple network cards, each one has its own address (for example, one for cable connection and the other one for wifi) which means that one device can appear under multiple MAC addresses on one network.
From a technical point of view, the network card prepares data to be sent and sends them to another device on the network. Also, it controls the data flow between the device and the network. The network card communicates on multiple layers of the so-called OSI model: it sends physical signals at the physical layer, data packets at the network layer, and operates as a network interface at the TCP/IP layer.
How does a network card look like?
Most of the times, NIC is a part of the device, ie. it is integrated right into the mainboard or is connected to it (via PCI, PCI-e or ISA interface). It can equally be an external device connected via USB cable, for example.