A person would go in to a place with a large computer connected to the Internet, usually a University or research lab., and sit down in front of one of the 'terminals' (not personal computers, just screens and keyboards wired into the 'mainframe' computer).
Then s/he would log on (put his/her name and password into the log or record, to show when s/he started using the computer that day), and a message would often flash up on the text-only (writing only, no pictures) screen:
you have mail
and the person could type a single key to choose (from a menu) to read the mail, one or more messages from other people also connected to the computer network. While reading a message, the person could choose to save it, print it, delete it or reply to it.
When the person had finished working on that terminal, s/he would log off, so the system had a record of how long s/he was using it.
In this way, anybody connected to the original Internet (major University and research computers linked together through modems and telephone lines) could send messages to anyone else on that network. It was fast (text-only and not many connections), and in those days, free for the users.
And of course, it is no longer free to users.
However, much of the main network is still maintained and run at their own cost by universities and research organizations, who still offer free accounts to their students and workers.
What you, the average consumer, pay for (not forgetting your own phone calls to your ISP) is connection time, the cost of using the equipment and telephone lines which your Internet Service Provider must buy and rent and continually update to make it possible for many of you at any time to be connected to the main Internet.
Some ISPs will sell you a whole year's connection in advance, for around $1000 I think.
This makes me nervous, because who can predict what will happen in the next year?
(Paying out large amounts of money for something in advance always makes me nervous ;-)
Other options are paying a bit ahead as you go (cheaper for you and for them, because they don't have to chase you for it), and paying an account monthly or by direct debit.
Some ISPs have 'plans' like the mobile telephone 'plans', where you estimate how many hours connection time you will use.
If you can estimate accurately, this can save you money, too.
next article: Connecting to the 'Net