I've read at the open source barometer that it is possible to access the alfresco repository in two different ways, by browser or by portal. I am wondering which are the differences, I though alfresco is also a portal but now I am a bit confused about it. :?
Alfresco provides a Web Client, but it's not a portal, i.e. we don't host third party portlets.
As for why you would need a portal, that's more down to how you intend to use Alfresco. I suggest you have a look at the Alfresco Web Client and also at LifeRay and see which one fits your business needs better as a user interface.