For my next application i would like to implement something that has a feature like the facebook wall but let me explain a bit. For those of you who used facebook you know that when somebody posts a message on your wall, and you are logged in to your account, you will get a notification immediately somewhere in the lower left corner. Lately they even pushed this a little bit further and if somebody comments on it the comments are updated as you visualize the page, it's like an instant chat. 

My application will be developed in PHP, I will use Zend Framework to do it. I'm interested in the basic principle that makes the facebook wall behave like that (updates in real time). I know there is ajax involved but I can't really tell how is the javascript triggered when the user is doing something. Or even more, how to push back to a user some info that was added after he viewed the page. For example, let's say that a somebody adds me as a friend. I would like to see a notification saying "X has added you as a friend" if i am logged in. I hope you understand what I'm trying to do.
If you can tell me some basic ideas, maybe provide some links that have this information I would be very grateful. 

Thank you for your time in reading this.