What is FFmpeg?

FFmpeg is the leading multimedia framework to decode, encode, transcode, mux, demux, stream, filter and play. All builds require at least Windows 7 and are licensed as GNU GPL 3.0.

Some Useful Commands

• ffmpeg -i video.mp4 -i audio.aac -c copy output.mp4 (combines input files into a single file as output.mp4)
• ffmpeg -i video.mp4 -i audio.aac -c:a ac3 -c:v copy output.mp4 (Re-encodes input aac to ac3 and combines input files into a single file as output.mp4)
  
• ffmpeg -i video.mp4 -vn output.ac3 (Extracts audio from input file as output.ac3)
  
• ffmpeg -f concat -safe 0 -i list.txt -c copy output.mp4 (combines files that specified in the list.txt (format per line: file 'full path') into a single file as output.mp4)
  

FFmpeg website