You can link two computers directly together using 2 network cards and a "Crossover" cable.
If you want to link more than 2 computers together then you can put a network card in each of them use a switch and a "patch" (cheaper than crossover) cable. A switch is a big box with slots for network cables to plug into, so that you can have more than 2 computers talking to each other.
A hub is like a switch. Its slower but cheaper
Across either type of network you can share files, printers, internet etc...
Gigabit is faster but you need fastr (gigabit) cables and network cards (which are more expensive).
For home use you dont really need gigabit but you can use a gigabit switch with all your old stuff though. Its backward compatible, it just will run at the lower speed.
The WAN port is for plugging your broadband internet connection into so that you can share it between all the pcs plugged into it. These are on things called "routers". A router is like a switch but costs more.
In the long run a router is better because its easier to share the net and it offers a built in firewall etc....
just get a switch, some network cards (you might have network ports built into your motherboard, they look a bit like phone line sockets) and some Patch cables (if you have a switch you dont need the crossover ones).
Plug them all in then the real fun begins. It all depends on which OS you have. XP is the easiest to configure.
each device will need a unique IP adress. Go into control panel -> network connections. Right click on the network card (should say something like "Local Area Connection") and choose properties.
Select "Internet Protocol (TCP/IP)" from the list. Click properties.
Once its restarted right click on the desktop and do properties -> desktop -> customize desktop.
Make sure "My Network Places" is checked. Now double click the my network places icon which has just appeared on your desktop. In the toolbar on the left there is a button to run the Home Networking Wizard. Run that. MAKE SURE THAT ALL PC'S ARE IN THE SAME "workgroup" (MSHOME is a good choice). The wizard will ask you whether to share the shared documents folder, printers etc....
To share mroe directories right click on them in my computer and select Sharing and Security