(The following diagram is taken from http://help.sap.com - an official SAP Help Library.)As you can see from the daigarm, our web browser will be the client, and the portal server that contain the Portal Framework - I call it as prt (portal runtime) framework will process all the request of the client.
SAP EP will provide a Single-Sign-On access to the clients through the portal to access all the backend systems / applications and enterprise resources in a secure channel (authorization and authetication process will be taken place during the sign on process).
In prt, it contains three main areas, which are : Portal Components , Portal Services, and Connectivity Framework. As a developer who develop portal application that deploy into the portal server, these are the components that the developer will be developing.
A portal component is custom Java code that is executed according to user requests, and generates HTML output for display on the client. It is a reusable component that can be combined with other portal component in another portal application.
A portal services act as the middleware, that is, interfaces that are enabled to exchange procedures and data. They offer functionality to portal components, and other services.
Connectors Framework enable integration between Enterprise Information Systems (EIS) and application servers and enterprise applications in the Portal Platform.
I will further explain the main features and functionalities of these components.
The bottom part will be the "Information Source" - it can be any data source systems like SAP System (i.e. BW , CRM, SEM and etc) , Business Information System, databases, and any resources (i.e. documents and applications that sit in the backend)