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Computer network - A collection of computing devices that are connected in various ways in order to communicate and share resources.
Types of networks: 
· Local-area network (LAN) A network that connects a relatively small number of machines in a relatively close geographical area. 

· Wide-area network (WAN) A network that connects two or more local-area networks over a potentially large geographic distance. 

· Metropolitan-area network (MAN) The communication infrastructures that have been developed in and around large cities.

Various configurations, called topologies, have been used to administer LANs
- Ring topology A configuration that connects all nodes in a closed loop on which messages travel in one direction

- Star topology A configuration that centers around one node to which all others are connected and through which all messages are sent

- Bus topology All nodes are connected to a single communication line that carries messages in both directions.

Twisted-Pair Cables: If the pair of wires are not twisted, electromagnetic noises from, e.G., motors, will affect the closer wire more than the further one, thereby causing errors.

Unshielded Twisted-Pair (UTP) : Typically wrapped inside a plastic cover (for mechanical protection). A sample UTP cable with 5 unshielded twisted pairs of wires.

Shielded Twisted-Pair (STP): STP cables are similar to UTP cables, except there is a metal foil or braided-metal- mesh cover that encases each pair of insulated wires.

Coaxial Cables
- In general, coaxial cables, or coax, carry signals of higher freq (100KHz–500MHz) than UTP cables

- Outer metallic wrapping serves both as a shield against noise and as the second conductor that completes the circuit.