Using K2, you can transform your Joomla! website to a news/magazine site with author blogs, product catalogs, work portfolio, knowledge base, download/document manager, directory listing, event listing and more, all this bundled under one package! Since K2 is extensible with additional fields to its base item form, you can easily create category-specific content types, e.g. article, blog post, product page, directory listing.
this is from
http://k2.joomlaworks.gr/
These days nearly everybody is promoting K2, must be some kind of marketing trick ...
This component is working pretty well out of the box, you don't have to do much custom work like you have to do with other similar components like SOBI2
http://www.sigsiu.net/sobi2.html
, ZOO
http://zoo.yootheme.com/
, or jSeblod CCK
http://www.jseblod.com/joomla-cck.html
BUT: working with these type of components is a one way route (except jSeblod which uses joomla database tables). These components are creating new database entries, that means, 90% of the modules/plugins/component for joomla won't work with them! That means problems with SEF components, you can't create working menus out of the joomla menu manager ...
In K2 you can import your joomla-articles but once they are stored in K2 that's it, no way to get them and new K2 articles back to joomla, once you decide to quit with K2 (think of upcoming joomla 1.6 with lots of changes), without working with tools on database level. There are some questions related to that topic, but they are never answered at the joomlaworks forum.
Btw, support at joomlaworks is poor. Eveybody is twittering around and promoting other (commercial) stuff but too many unanswered questions in their forum. For the bigger probs, they suggest to pay for commercial support at "open source support desk". That's the direction joomla community is taking...
There's other components to modify the output of joomlas content f.e. something like alphacontent
http://testing.visualclinic.fr/alphacontent.html
is doing a good job displaying directories and using joomla database tables for content...