What is the difference between application server and web server?
It depends on the specific architecture. Some application servers may use web protocols natively (XML/RPC/SOAP over HTTP), so there is little technical difference. Typically a web server is user-facing, serving a variety of content over HTTP/HTTPS, while an application server is not user-facing and may use non-standard or non-routable protocols. Of course with RIA/AJAX, the difference could be further clouded, serving only non-HTML content (JSON/XML) to clients pumping particular remote access services.