As for the Architecture, it is quite simple: WebCenter Portal is a J2EE application built on top of the ADF framework.
It has too many features to be listed, but the initial introduction can be based on the wikipedia definition of an enterprise portal, namely:
- secure unified access point (your users will always start here and based on their roles they will have access only to applications/information they are authorized to use)
- aggregate (e.g. data from one application can be enriched by data from the other app or public resource without necessity to integrate at the application/database level)
- personalize (users, or group leaders are authorized to modify layout/content of "their" portal pages without affecting other users)
The advantage of WebCenter Portal is also that
a) it integrates smoothly with Oracle Fusion Applications (front-ends of these apps is written in WebCenter Portal)
b) it provides many useful "2.0" services out of the box
Don't hesitate to ask if you have further questions.
If you want to have a clear understanding on the architecture, please read the enterprise deployment guide: http://docs.oracle.com/cd/E23943_01/core.1111/e12037/toc.htm
This described all the components in much detail. It gives also a detailed overview of the architecture.