Basically you have to look at a network this way.
You are going to have some sort of "wire" coming from a service provider that gives you internet access, I will leave phone lines out because a majority of it is coax cable from your cable tv provider or now even fiber optic cables right to your home / business. Now when that comes in from the outside its going to go into a box could be a cable modem or a router, but the service provider will provide that. So I am only going to talk about the network inside from that box becuase that is where the NIC card operates.
So in order for a computer or multiple computers to reach the internet through that box they need to be connected together using something, which is usually ethernet cable which is CAT5, CAT5E, CAT6 with a RJ-45 plug on it.
So what you do is take either a hub, unmanaged switch, or managed switch and place it on your network. So you connect your internet box to the WAN port of that with an ethernet cable. Now you have to run ethernet cables from that hub or switch to the local PC, you do this also with an ethernet cable and that plugs into a NIC card.
What the NIC card does is plug into usually a PCI slot in your PC and converts the information from your PC to the actualy electrical impulses that will travel across the wire. It is the bridge between the cable network and the information on your computer.
This is where the OSI model comes in handy. http://www.3mfuture....el-7-layers.png
That link is a picture of what I am talking about. The NIC card is at the very bottom. Thats basically what a NIC card does.
Now there are more than just ethernet NIC cards and ethernet cable, but this is the most common method and easiest to explain.
If you have any questions on any of this let me know and I will clarify it for you.