Many applications in Ubuntu use the GStreamer open source multimedia framework. The codec plugins for GStreamer are organised in different packages depending on the licensing that applies to the various codecs. You can see which codecs are contained in which plugin on the GStreamer website.
Other applications, such as Mplayer and Xine, do not use the Gstreamer framework. Due to patent and copyright restrictions, codecs for these programs are not included at all in Ubuntu. For more information on these read the Restricted Formats wiki page.
* License
