How, if possible, can you chat using talk or write in the app Terminal?
The command is just talk. It's disabled in OSX by default. See the comment at the end.

eg.

talk alex


It will then open a split screen and ping the other person to chat to you. They have to be logged into your computer at the same time though. Given that most Macs are single-user, this is unlikely, but possible. Have a friend connect in via SSH or screen-sharing and give it a go.

write allows you to write straight onto the other users console. By default though users have it disabled. You can turn it on with 'mesg y'.

write alex


Then type away to irritate the other user.

These are pretty old unix commands. Been around on linux/UNIX for years and years.

The man pages will help you too:

man write
man talk
man mesg


You can test this on one computer by creating a different ID and opening up Terminal with two windows. In the first window log back into your box with the other ID:

ssh otherid@localhost


Then write to the main users Terminal:

write mynormaluser

eg:

write alex


You'll see the text appear in window 2 of the Terminal

To get talk to work on OSX you need to enable it first. As root run this:

launchctl load -w /System/Library/LaunchDaemons/ntalk.plist


or, as a super user:

sudo launchctl load -w /System/Library/LaunchDaemons/ntalk.plist