Scald (SCALD is Content, Attribution, Licensing, and Distribution) is a different take on how to handle media (audio, video, image or otherwise) in Drupal. It defines a new entity type, a field type and a few concepts of media like player, transcoder, unified type.
Have a glance at README.txt to catch up with Scald in 5 minutes.
While 6.x versions are actually used in production in many sites, new development focuses on 7.x branch. If you want to test, or to use Scald in your new website, 7.x-1.x is the obvious choice. If you want to start a new Drupal 6 site, you can use the 6.x version, but please don't expect for a stable release.
These pages are mainly for 7.x version.