I really like the notification system that Stack Overflow has in place. For example, when I am typing in an answer to a question and someone else answers that same question, an orange notification drops down at the top of the screen letting me know that this has happened.
I would like to implement a similar feature in my own site but could use some help understanding how this works. Specifically, it is fascinating to me how the notification can occur without any client side action by the user. I was going to ask the question of how this works on Stack Overflow but realized that the team at Stack Overflow might not like a trend of developers trying to reverse engineer all the features of Stack Overflow on the site. Is this an acceptable question? It's not like I am trying to develop another Question & Answer site.
EDIT: It was difficult to find but I just found a question asking exactly what I am curious about here.