A portlet application is a standard application with an additional file:
WEB-INF/portlet.xml

Two versions of the portlet specification exist at the moment, we can simply focus on the second one.

The schema for the first version of the portlet specification is available at:
http://java.sun.com/xml/ns/portlet/portlet-app_1_0.xsd

The new version should be available at the following address anytime soon:
http://java.sun.com/xml/ns/portlet/portlet-app_2_0.xsd