Fedora Core is in rapid development. Repositories or in short repos (plural of repo) are servers which provide packages for Fedora Core users to install and update packages. For those who likes to live on the edge should enable testing repo. For newbie, you can use pirut or go to Application --> Add/Remove Software and enable the testing repo. Be warned that the packages provided are not stable. They can even make your computer system hang and your CD drive burnt ;). I don't like it. I like everything to be stable and I have experienced how it was to be living on the bleeding edge. Things were not that good. :D
Beside core, updates and extras repositories, you might need to add livna and kde repositories too. These sites provide non-free/proprietary packages like mp3 support and proprietary display drivers (ATI and NVIDIA). The kde repo stated above provides unofficial kde packages from kde community itself. The packages comes as updates for the packages provided by official Fedora repo.To add livna repo, run this command :
rpm -ivh http://rpm.livna.org/livna-release-6.rpm
To add kde repo, download this file using wget:
and put this file in /etc/yum.repos.d/
Afterthat, try checking updates by running yum check-update. You should see many updates come from both repos.