A lot of camcorders record in the .mov (Quicktime) format. It has high quality video and depending on the underlying codec it provides a good compression, but it is too proprietary and very often you might not be able to play it on different devices.
AVI on the other hand is more widely accepted format. AVI, like MOV, is a media container that envelops encoded media. In this example we will convert Quicktime video (.mov) to .avi that contains media encoded with mpeg-4.
If you want to preserve the original quality and resolution with pcm_u8 (analog 8bit) audio:
ffmpeg -i inputfile.mov -sameq -vcodec msmpeg4v2 -acodec pcm_u8 outputfile.avi
Obviously I would prefer something better for the sound so I normally use the mp3 as an audio codec. To do this make sure that you have the libmp3lame0 package installed to do that:
sudo apt-get install libmp3lame0
ffmpeg -i inputfile.mov -sameq -vcodec msmpeg4v2 -acodec libmp3lame outputfile.avi
If you want to change the resolution then add the -s option and specify the horizontal and vertical resolution. Make sure to preserve the original aspect ratio:
ffmpeg -i inputfile.mov -s 960x540 -sameq -vcodec msmpeg4v2 -acodec libmp3lame outputfile.avi