There wasn't such thing as inter-system operability. In short, if you were using a Mac, there is no way you can talk to another person on Windows. (since the internet/email wasn't really there yet) Alright this is a bad example, let me write a short summary of what it's like back then.

You have something called a mainframe, or workstation. Computers are really expensive then, so multiple users would share 1 computer. (imagine multiple users logged into your computer and using it at the same time, but not directly affecting each other) Each user has a screen that connects to the computer, and they could execute commands to it. Of course, the computation powered is shared among the number of users. And the wonderful Operating System is called Unix. 

(Windows/Mac OS are created afterwards for user-friendliness)

During the era that doesn't have internet, how would you chat with another user on the same computer?

Introducing, Talk.

(such a creative name)