The most important PC device is the network interface card (NIC). Each computer on the network, including the servers, is required to have one installed. It is the NIC that provides connectivity between the PC and the network's physical medium, the copper or fiber-optic cable.
Most of the new motherboards available today for PCs and servers have the network interface card integrated with the motherboard. Older computers and some newer computers do not provide onboard network interfaces which will equire a NIC to be added.
NICs provide computers with a connection to the network, but they also handle an important data-conversion function. Data travels in parallel on the PCI's bus system, but the network medium demands a serial transmission. The transceiver, a transmitter and receiver, on the NIC has the ability to move data from parallel to serial and vice versa. This isn't any different than an automobiles travelling down a multi-lane superhighway where all lanes must merge into one lane.
Network interface cards also have the ability of supplying a basic addressing system that can be used to get data from one computer to another on the network. The hardware or MAC address is burned into a ROM chip on the NIC. This is referred to as the MAC address because the Media Access Control (MAC) layer is acutally a sublayer of the OSI model's Data Link layer.