[ excerpt from developer's site ]
- What is GStreamer?
GStreamer allows the construction of graphs of media-handling
components, ranging from simple mp3 playback to complex audio
(mixing) and video (non-linear editing) processing. Applications
can take advantage of advances in codec and filter technology
transparently. Developers can add new codecs and filters by writing
a simple plugin with a clean, generic interface. GStreamer is
released under the LGPL, with many of the included plugins retaining
the license of the code they were derived from, usually GPL or BSD.

- Features:
* Comprehensive Core Library
* Intelligent Plugin Architecture
* Extensive Development Tools

- Is GStreamer a media player?
No, GStreamer is a development framework for creating applications
like media players, video editors, streaming media broadcasters and
so on. That said, very good media players can easily be built on
top of GStreamer and we even include a simple yet functional
mediaplayer with GStreamer called Gst-Player

WWW: http://gstreamer.freedesktop.org/