How do you write a real-time webbased collaboration tool such as google docs?

I'm really curious about

How one would go about writting a collaboration tools such as google docs?

The closest thing that I can think of is to write a messaging system like a chat and then send messages back and forth so that each client updates itself.

Do you know of a tutorial / demo where they program a small tool like this?

What tools do they use to do this kind of apps?

TIA for any pointers you can provide.