both 'wall' and 'write' are commands that you can issue through a terminal. The best way to get information is to read the 'man'-pages. Open a terminal and type 'man wall' or 'man write' and you will get all the information that you need.
To make it easy, I can narrow it down here for you (but then you have to promise to adopt a kitten and name it after me!)
Open a terminal and use the command 'who' to see who's online:
wim tty7 2011-04-15 00:28 (:0)
nerd tty3 2011-04-14 13:46 (:0)
geek tty2 2011-04-15 01:02 (:0)
wim pts/0 2011-04-15 00:39 (:0.0)
You can see that I (wim) am both logged in through terminal 7, that I am also straight at the computer (pts/0), and that there are two other users ('nerd' and 'geek').
If I want to write something to 'nerd', then I just issue the command:
wim@nerdypc:~$ write nerd
and from then on, whatever I type will appear in nerd's terminal session. Every time I press 'enter', the string will be sent to nerd. To stop transmitting, press CTRL+D .
Nerd will have received something as follows:
Message from wim@nerdypc on pts/0 at 00:45 ...
this is ye matey wim, how art thee?
Now, the command wall is to broadcast to all users currently logged in on a terminal session of the computer. So to broadcast (you might need to be superuser, depending on the user regulations), I do the following:
And I can start typing whatever I want, just a short sentence or even a full poem if I'd like. The message won't be sent until I end it all by pressing (again) CTRL+D . Only then, both 'nerd' and 'geek' (and whomever has also logged in by now) will see the following on their screen:
Broadcast Message from wim@nerdypc
(/dev/pts/0) at 00:45 ...
prepare to activate and take over ze vorld.
So, to re-cap: 'write' is to send a message to a single user and the message gets sent every time you press the 'enter' button. 'wall' is used for broadcasting and the complete message gets sent at once at the end.
Besides that, I used to know of the command 'talk' . The recipient has to issue a command as well to accept the talk-invitation and then you will be in some sort of inbuilt chat-system that takes up the whole screen, one user can (in realtime) type at the bottom, the other at the top-half.
Hope this helps. Kind greetings,