Normal Web Application Versus Portlet Application

Here are few conceptual differences...

1. Portlets reside in a Portlet Container where as J2EE Web App requires Servlet container.
2. Portlet provides only a fragment of the content on a Portal Page where as Web App is responsible for the entire content of the page.
3. Any request to a portlet is intercepted by the portal first which then dispatches the request to particular portlet, where as any request to web app is interceped by the servlet container which then forwards the request to relevant servlet.