An open source media framework for building graphs (think DirectX if you know it - you connect a number of inputs (cameras, media files etc) through a number of filters, multiplexers etc, to a number of outputs) in media applications.
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. including most of their plugins. It is based on the GLib 2.0 object model for object-oriented design and inheritance.
A good example of a GStreamer front end is Totem.