Your computer and your cable modem are Ethernet endpoints. Sometimes they have Ethernet ports built in (they take Cat5 cable), other times you will need to by an Ethernet card (also called a Network Interface Card, or, redundantly, a NIC card). Each Ethernet endpoint has a globally-assigned identifier, called an Ethernet Address (also called a MAC Address). Example:
00:30:65:f3:76:b6. Portions of the address space are assigned by the IEEE to hardware manufacturers, so you can distinguish between manufacturers by these addresses.
Two Ethernet endpoints can communicate with each other by using a crossover cable. These are special types of Cat5 cables, usually read, which have the transmit and receive wires reversed.
People used to "daisy-chain" machines together into Ethernet networks using coaxial cable but this is mostly obsolete.
To connect together more than two endpoints, you plug each endpoint into a port on a hub. A hub takes every packet it receives on port and transmits it on every other port. Note that all endpoints receive all traffic.
To connect hubs together, you can use a crossover cable between two ports or you can use a special uplink port which functions in the same fashion as a crossover cable.
When doing this, you must be careful to prevent loops, where one hub feeds into a second which feeds back to the first. This quickly creates a lot of feedback and disrupts traffic.
Switches function like hubs, except that they look at the packets and see which Ethernet addresses are located on each port. Then they only retransmit packets to the relevant ports.
Switches also have spanning-tree protocol, which prevents loops by making sure switches are connected in a tree formation.
@@What's the protocol?