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A CMS stands for Content Management System and while many use that term to describe most web applications it actually represents more content oriented web systems for managing static or alike contents that are on your website.
Portals on the other hand are systems dedicated more to the noding of content which is usually dynamic instead of acumulating and managing it on site.
However, many portal sites are actually both content and portal sites at the same time.
I think that XOOPS can provide for both, but is mostly for community oriented portal sites that are typically forum centered with additional content in form of articles, news, reviews etc. And then, it can be a pure portal with the headlines, links, RSS feeds, web digest, polls etc. that are more noding the existing contents from other sites than are generated from the portal site itself.
Thank you
Daniel