A computer network is just a set of stuff for nodes to talk to each other (by `nodes' I mean computers, printers, Coke machines and whatever else you want). It doesn't really matter how they are connected: they could use fiber-optic cables or carrier pigeons. Obviously, some choices are better than others (especially if you have a cat).
Usually if you just connect two computers together, it's not called a network; you really need three or more to become a network. This is a bit like the word `group': two people is just a couple of guys, but three can be an `group'. Also, networks are often hooked together, to make bigger networks; each little network (usually called a `sub-network') can be part of a larger network.
The actual connection between two computers is often called a `network link'. If there's a bit of cable running out of the back of your machine to the other machines, that's your network link.
There are four things which we usually care about when we talk about a computer network:
If you simply connect your four computers at home together, you have what is called a LAN (Local Area Network). If everything is within walking distance, it's usually called a LAN, however many machines are connected to it, and whatever you've built the network out of.
The other end of the spectrum is a WAN (Wide Area Network). If you have one computer in Lahore, Pakistan, one in Birmingham, UK and one in Santiago, Chile, and you manage to connect them, it's a WAN.
Draw a map of the network: lines are the
network links, and each node is a dot. Maybe each line leads into a central node like a big star, meaning that everyone talks through one point (a `star topology'):
o o o \_ | _/ \|/ o-----o-----o _/|\_ / | \ o o o
Maybe everyone talks in a line, like so:
o------o------o-------o--------o | | | | | o | | o | o
Or maybe you have three subnetworks connected through one node:
o o | o--o--o | | | o--o--o--o--o o \ | o------o / | o--o--o--o--o o | | | o | o--o o
You'll see many topologies like these in real life, and many far more complicated.
The second thing to care about is what you've built the network out of. The cheapest is `sneakernet', where badly-dressed people carry floppy disks from one machine to the others. Sneakernet is almost always a LAN. Floppies cost less than $1, and a solid pair of sneakers can be got for around $20.
The most common system used to connect home networks to far bigger networks is called a `modem' (for MODulator/DEModulator), which turns a normal phone connection into a network link. It turns the stuff the computer sends into sounds, and listens to sounds coming from the other end to turn them back into stuff for the computer. As you can imagine, this isn't very efficient, and phone lines weren't designed for this use, but it's popular because phone lines are so common and cheap: modems sell for less than $50, and phone lines usually cost a couple of hundred dollars a year.
The most common way to connect machines into a LAN is to use Ethernet. Ethernet comes in these main flavors (listed from oldest to newest): Thinwire/Coax/10base2, UTP (Unshielded Twisted Pair)/10baseT and UTP/100baseT. Gigabit ethernet (the name 1000baseT is starting to get silly) is starting to be deployed, too. 10base2 wire is usually black coaxial cable, with twist-on T-pieces to connect them to things: everyone gets connected in a big line, with special `terminator' pieces on the two ends. UTP is usually blue wire, with clear `click-in' phone-style connectors which plug into sockets to connect: each wire connects one node to a central `hub'. The cable is a couple of dollars a meter, and the 10baseT/10base2 cards (many cards have plugs for both) are hard to get brand new. 100baseT cards, which can also speak 10baseT as well, are ten times faster, and about $30.
On the other end of the spectrum is Fiber; a continuous tiny glass filament wrapped in protective coating which can be used to run between continents. Generally, fiber costs thousands.
We usually call each connection to a node a `network interface', or `interface' for short. Linux gives these names like `eth0' for the first ethernet interface, and `fddi0' for the first fiber interface. The `/sbin/ifconfig' command lists them.
The final thing to care about is the language the two are speaking. When two modems are talking to each other down a phone line, they need to agree what the different sounds mean, otherwise it simply won't work. This convention is called a `protocol'. As people discovered new ways of encoding what the computer says into smaller sounds, new protocols were invented; there are at least a dozen different modem protocols, and most modems will try a number of them until they find one the other end understands.
Another example is the 100baseT network mentioned above: it uses the same physical network links ( UTP) as 10baseT above, but talks ten times as fast.
These two protocols are what are called `link-level' protocols; how stuff is handed over the individual network links, or `one hop'. The word `protocol' also refers to other conventions which are followed, as we will see next.