The Internet originated in the late 1960s when the United States developed ARPAnet - Advanced Research Projects Agency network - a system of computers able to survive, even if partially, after a bomb attack.
That system was made by a lot of computers connected together, like a web.
That web was divided in LAN - Local Area Networks.
Those computers were able to communicate with each other even is some of them were out of service. From that time the Internet is by now world wide, or WWW - World Wide Web
The protocol that rules all these computers is TCP/IP:
TCP = Transmission Control Protocol
IP = Internet Protocol
The wonderful characteristic of this protocol is the way it deals with the information sent and received. The normal phone transmission works in this way:
From point A directly to point B, using cable or radio connection. So if something wrong happens to the radio transmitter or to the cable, the communication is impossible.
With the TCP/IP protocol, on the contrary, the connection from A to B is not direct.
The information (file) that we want to send is divided in several parts, packets, and each packet has a bit of the whole information.
Each packet has a header and a footer, where are store all the indication for delivery what is being sent (Address, sender, number of the packet).
These packets follow different paths of the net, passing through different computers, so even if some of them are not working, the file uses the ones which are working properly. The computers may be all over the world. After this trip the file is rebuilt in the right way.
For instance, if we send a file to a computer that is in a house close to our office, may be that before arriving to destination, it goes to Europe and back.
To try to understand just a bit this system, we can do this very simple example:
We want to send a letter to someone. Instead of sending just one sheet of paper, we could send 6 sheets. Each one contains 1/6 of the whole letter.
On the 6 envelopes we write the sender, the address of the receiving person and the number of the letter (Letter n. 1, letter n.2, and so on).
Then we can send all this letters, from several Post Office or from just one.
The person who gets all these letters, has to follows the number on each letters (1,2, 3...) to be able to read the whole message.
This is good also for safety: if someone gets some letters, he has only part of the message not the whole message.