There was a pretty good comment before here in the list. Just wanted to clerify the concepts even more.
If you just listen to the names.
The name Portal (http://en.wikipedia.org/wiki/Portal) suggest a doorway or gate to something.
This is true since Portals themself doesn't contain anything.
It could however contain a CMS to display content. Other then that it just contains links, navigations and users.

There are some priciples that a portal tries to solve.

* Personalization - a portal should be able to let the user do personalizations. For instance a user could be able to add or remove portlets to a specific portal page.
* Internationalization - the portal comprises a platform for handeling different languages based on user settings, or other settings.
* Authorization - the portal handels the concepts of users, groups and roles. Authorizing a user gives some predifined role in which the user can access relevant functionality.
* Integration - the portal solves the person integration by enabeling views to multiple system at once using SSO to all systems. This is perhaps the most important feature of a portal since the users need a quick overview of information stored in different systems.

I've might have missed some concepts but I hope you get the idea of what a portal is. All this concepts where described by Gartner in order to determine differend generations of portals.