A network interface unit connects a network node to a link. Each network interface unit has a world-wide unique MAC address.
Hubs interconnect network nodes at the physical level. They just forward packets coming in on one of their interfaces to all other interfaces.
Switches interconnect network nodes at the data link level. They use data link addresses (MAC addresses) to forward packets only to the interface that connects to the packet destination.
Routers interconnect network nodes at the network level. They can make complex routing decisions based on packet internet addresses.