Online programming

I want to learn how to write a program that lets several users interact with the same content, online. Examples include a multi-user text editor, a chatroom, or an online game. What do I need to learn? What should I read?

There seems to have been a minor misunderstanding. I wish to learn how to write such programs, rather than use such programs (though I certainly can learn a thing or two from those of the above which are opensource).