Humans and animals communicate with each other in order to share some kind of information. On the early stages of human development it could be information about food or hunting. They communicated with the help of speech or sounds. Later came signals which enable humans to communicate over distance, fire and drums are the most widely known.
Over the centuries more and more methods have been developed using the technology available. For the most part of the 20th century the prevailing methods of communication were telegraph, telephone and mail. They were used for both personal and business communication. Though quite efficient and fast they were not fast enough for the modern world with the message and reply being spread in time.
Computers offered an alternative which provides fast and efficient transfer of data in the form of a message, sound or a graph. The data is transferred instantaneously and the information is the most up-to-dated. The idea of time and distance isn't an issue with the modern technology. Thus, from being a machine for personal use, the computer has become a communication tool.
However, to be able to transfer data we have to link the computers into a network. There are different kinds of networks which serve different purposes but the same principle of data transfer is used in all of them.
Communication occurs between a sender, a receiver via a medium that carries the data:
sender --- medium --- receiver