FFmpeg (or "Fast Forward MPEG") is a simple yet feature rich command line utility to allow the manipulation (including decoding, encoding, transcoding, muxing, demuxing, streaming, filtering, and playing) of media and video files. This utility differs from other GUI orientated software as it employs the WYSIWYG methodology (What You See Is What You Get). Instead of hidden away menus and features, everything can be found by just typing
ffmpeg -h when set up, or following the comprehensive documentation. In addition to the command line tool there are a number of C libraries (which it uses), that can be used to bring the functionality of FFmpeg into other projects. The documentation for the libraries includes many examples to help get you started.