The packages are categorized according to their stability.

• stable packages imply minimal upgrading risk. They are considered very well tested.
  You will usually find slight modifications to Red Hat's own packages here.
• good packages have been working very well, but there hasn't yet been enough third party feedback to judge on more setups.
• testing packages are provided on a "works for me" basis.
• bleeding packages means asking for trouble!
• obsolete packages will be removed soon.

The stable, good and testing packages are enabled by default in apt-get and yum. bleeding packages can be enabled in the respective configuration files, while obsoleted packages are offered only for manual download.