[ excerpt (with adaptations) from developer's README ]
ffmpeg is a hyper fast realtime audio/video encoder, a streaming
server and a generic audio and video file converter.

It can convert a standard video source into several file formats
based on DCT/motion compensation encoding. Sound is compressed in
MPEG audio layer 2 or using an AC3 compatible stream.

What makes ffmpeg interesting ?
- Simple and efficient video encoder: outputs MPEG1, H263, Real
  Video(tm), MPEG4, DIVX and MJPEG compatible bitstreams using the
  same encoder core.
- Hyper fast MPEG audio layer 2 compression (50 times faster than
  realtime on a K6 500).
[snip -> rest on website below]

ffmpeg is made of two programs:
* ffmpeg: soft VCR which encodes in real time to several formats.
  It can also encode from any supported input file format to any
  input supported format.
* ffserver: high performance live broadcast streaming server based
  on the ffmpeg core encoders.

WWW: http://ffmpeg.sourceforge.net/
