I've done my reading on portlets only recently so please forgive my ignorance.
Is it correct to associate portlets with an HTML Document? Portlets seem to me like just a bunch of page includes although I'm quite aware that something else is happening on the server-side... I'd just like to ask if all the portlets on a single page only equate to a single HTML document?
Yeah, when you get a portal page, it's delivered to the client's browser as a single, simple, HTML file, with one start <HTML> tag and one </HTML> tag.
Each portlet generates just a snippet of code to be displayed,usually manifesting itself as a html <TABLE> behind the scenes.
Indeed, you can think of the portal as an aggregation service, aggregating or putting together all of the portlet snippets, very possibly with a jsp:include, and then throwing on a theme for the page, and sending everything back to the client.
So yes, it is pretty complicated as to how the portal figures out user rights, permissions, personalization, customizations, language preferences and the like, but in the end, it's all just a simple HTML page that the client gets.