A portal is generally view tier software gathering information from many sources and re-purposing it, for example from Liferay's website "A portal is generally defined as a software platform for building websites and web applications." A content management system like Alfresco on the other hand is really a back end "system of record".
So you have Liferay's UI bits on-top of Alfresco's content management, possibly best of both worlds.
However where things become blurred is that neither Alfresco without a UI or Liferay without content management are much use so Alfresco also has view tier features and Liferay has some content management features. That's where the products overlap.
And to complicate things further Alfresco also has WCM functionality.