In short, a call to ActionURL will return an entire portal page (with theme, renderers, and all), whereas a ResourceURL serves your markup raw but still provides access to the portal context.
Pre-JSR286, this was accomplished with backing servlets, which did not have implicit access to the PortletContext.
Thanks for your response.
My question is what could be the real usage scenario where I would like to serve a raw markup without the portal's theme, renders etc.
For the end user, if all of a sudden I show him a page without the Portal look and feel - he will feel that he has been logged out of the portal (though may be technically I'm within the Portal context).