Portal:-A portal is a web based application which aggregates the portlet applications in a presentable format. Portal provides Personalization, Single Sign On and content aggregation from different sources.
A portal typically allows users to customize their presentation, including what portlet applications to display on a portal page.
Portal server implementations include a component receiving requests and aggregating personalized content for the respective user.
Typical portal server content

Portlets:-
A Portlet is a web based java component managed by portlet container much the same way as servlet is managed by the servlet container.
But portlets have additional, special properties that allow them to easily plug into and run in enclosing web applications like portals.
Portlet generates a fragment of markup that user can see from his or her browser.
User issues a request against a portlet from the portal page which in turn forwards this request to portlet container which manages portlet’s life cycle and then returns response to the portal and the portal which is finally responsible for aggregating the content produced by the portlets.
Portlets are not directly tied to a particular URL , instead they constructs URL using the methods like createActionURL() and createRenderURL().
Portlet Container:-:
As of now I have used the term Portlet Container so many times , but now it Is time to learn about what exactly portlet container is?
Portlet container sits between Portal and its portlets.
Portlet Container is responsible for running the portlets and provides them with required runtime environment. Portlet Life cycle is managed by the Portlet container.
Basic portal architecture

An Example which will explain the roles played by the above players(Portal,Portlet- Container,Portlet).
User issues a request for a particular portlet on a portal page using any client device such as browse or web enabled phone.
This request goes to portal which examines the portlet to be executed from list of portlets to satisfy the request.
Here the portal invokes the portlet through portlet Container which in turn runs the requested portlet and returns response to the portal.Portal is then responsible for generating fragment which is to be shown on portal page back the Client.