The write is small utility to send message to another user over terminal. It does that by copying line from your terminal to another.
$ write rahul tty7
You have to give user name to whom you have to send message and terminal on which he is logged in. To reply back user should also run write command. When you are done type end-of-file or interrupt character.
If person is logged on more than one terminal then you can specify to which terminal it should go otherwise let write decide where it should go depending on shortest idle time. If you want to block someone from writing to your terminal then that could done with mesg command. I will talk about mesg command some other day by that time see man page for write for more info.