I have a question. Is it possible to make real-time interaction between two clients?
Example image:

So User 1 and User 2 are connected to a server.
What I want to do is to pass specific values from User 1 to Server and pass corrected (or the same) values to User 2 in real-time. The less it takes values to send from User 1 to User 2 through Server, the better it is. There will be several buttons and depending on what data has come to that client, I want to change elements' properties (enable/disable buttons, add text to Text Area or better Div or other similar tag and so on...) in the client.
So what I want to know is if it's possible to do it with very low latency (I know it depends on both users' internet connection) and what languages, technologies should I use. I guess I should get hang of AJAX for interaction with server, but I don't know how to make it listen the server for new messages, data, so it could grab it as soon as it arrives to the server. Also, what language/scripting should I use to change page elements' properties? Javascript or something other?
If anyone has experience with these type of PHP scripts, please tell me any information I would need to make my work more efficient and less buggy. Or just point me to any web page, I have time to read
Thanks