Today we hear a lot of news about AJAX. Actually AJAX has changed the architecture and style of web based applications. AJAX helps a lot. But DWR has made it simpler. We have already used it in our applications and it is great.
There are a lot of documents about DWR but I found below links useful:

here is one article from javaworld:
www.javaworld.com/.../jw-0620-dwr.html

and here is the site:
http://getahead.ltd.uk/dwr/