If I screencast something, than most of the time I do it with recordMyDesktop which produces .ogg-Videos. If you need to convert it so you can show it to your friend or upload it to a video hosting page like youtube or so you have to convert it into another format. I chose avi as format.
Simply fire up this command to convert your file:
mencoder out.ogg -o out.avi -oac mp3lame -ovc lavc
- out.ogg is the .ogg-file you want to convert
- out.avi is the .avi-file you want to create
- mp3lame is the used audio-codec (here mp3)
- lavc is the used video-codec (here libavcodec)
For more information you can look at the MPlayer manual page.