If you download YouTube videos (using youtube-dl for example) you may want to convert them to mpeg for people who may have troubel viewing FLV files (perhaps Windows users who aren’t very technical).
The easy command is:
ffmpeg -i ./input.flv -sameq -ar 44100 ./output.mpg
The -sameq tells ffmpeg to maintain the same video quality as the original and -ar tells ffmpeg set an audio sampling frequency of CD quality (44100). This works in reverse, of course.