If you seriously had a hard time determining what Joomla! is after looking at Joomla.org, I respectfully suggest that, as nice as Joomla! is as a content management system, you look elsewhere, as I’m afraid you are likely to have trouble operating it.
Many find that using WordPress, also available as a “one-click” install on DH, is a good “first step” into running content management type applications. While less “full-featured”, and oriented primarily toward “blogging”, WP 's content management features are much simpler to grasp (you work with “pages”) than are those of Joomla! for many.