To set up a network you need at least two computers (there's not much
point having a network if you've nothing to share your resources with).
Each computer needs to have a Network Interface
Card (like the one shown here). The Interface Card is installed
inside the computer in one of its special expansion slots.
In a simple network you just have to use a special cable
to connect the computers together via their interface cards. However
the cables must have the right sort of connector
to attach it to the interface card.
Special software is needed that
will allow the computers to communicate with each other. The network
can then be used to share a printer or modem, play games against each
other, or send messages.