Jon Downes has written a great introduction to Reverse Ajax and how it works in DWR 2.0. From the article:
"The problem is that web servers can't easily contact web browsers. For one thing firewalls will get in the way, and even if they didn't, browsers only listen for answers to questions they've asked. So how is the web server to get the message through?
Answering this question is the essence of reverse Ajax. It is the means by which updates can be pushed from the server to the browser. Sure, techniques for doing this already exist and have cool names (see below), but Reverse Ajax ties them into a neat package and puts them more readily at the disposal of the developer."
The Reverse Ajax article is well worth a read.