GStreamer is a framework for creating streaming media applications. The fundamental design comes from the video pipeline at Oregon Graduate Institute, as well as some ideas from DirectShow.
GStreamer's development framework makes it possible to write any type of streaming multimedia application. The GStreamer framework is designed to make it easy to write applications that handle audio or video or both. It isn't restricted to audio and video, and can process any kind of data flow. The pipeline design is made to have little overhead above what the applied filters induce. This makes GStreamer a good framework for designing even high-end audio applications which put high demands on latency.
One of the most obvious uses of GStreamer is using it to build a media player. GStreamer already includes components for building a media player that can support a very wide variety of formats, including MP3, Ogg/Vorbis, MPEG-1/2, AVI, Quicktime, mod, and more. GStreamer, however, is much more than just another media player. Its main advantages are that the pluggable components can be mixed and matched into arbitrary pipelines so that it's possible to write a full-fledged video or audio editing application.
The framework is based on plugins that will provide the various codec and other functionality. The plugins can be linked and arranged in a pipeline. This pipeline defines the flow of the data. Pipelines can also be edited with a GUI editor and saved as XML so that pipeline libraries can be made with a minimum of effort.
The GStreamer core function is to provide a framework for plugins, data flow and media type handling/negotiation. It also provides an API to write applications using the various plugins.
Specifically, GStreamer provides
an API for multimedia applications
a plugin architecture
a pipeline architecture
a mechanism for media type handling/negotiation
a mechanism for synchronization
over 250 plug-ins providing more than 1000 elements
a set of tools
GStreamer plug-ins could be classified into
sources: for audio and video (involves protocol plugins)
formats: parsers, formaters, muxers, demuxers, metadata, subtitles
codecs: coders and decoders
filters: converters, mixers, effects, ...
sinks: for audio and video (involves protocol plugins)
GStreamer is packaged into
gstreamer: the core package
gst-plugins-base: an essential exemplary set of elements
gst-plugins-good: a set of good-quality plug-ins under LGPL
gst-plugins-ugly: a set of good-quality plug-ins that might pose distribution problems
gst-plugins-bad: a set of plug-ins that need more quality
gst-libav: a set of plug-ins that wrap libav for decoding and encoding
a few others packages