In any multi-user system there are ways to communicate with other users. There are two category of communications programs. Some programs let you communicate in real time with other users that are currently logged in (chat). Other programs let you send messages to users that are not currently logged in that they can read later (mail).
This is a primitive Unix chat utility. If you'd like to chat with another user type a command such as
$ write logname
Everything you now type is sent to the other user's terminal. The other user can execute a similar command to send text to your terminal.
Be aware that the material to write to a user will appear on their terminal in the middle of whatever else they were doing. Don't be disconcerted if this happens to you.
To terminate a write session use the Unix end-of-file character: ^D.
This program enables or disables the ability of people to write to you. Use the command mesg n to disable writing and mesg y to enable it. If you just type mesg alone, the program will report on the writable state of your terminal.
These programs are all mail programs. They all read and send the same mail files so it doesn't matter which you use. Mail is the simplest. The pine and elm programs are the most sophisticated. The program named mail is available on every Unix system in the universe.
Discussion about these programs is beyond the scope of this document. I refer you to the online documentation for more information.