MOVs are the standard audio/video format for Macintosh. They can contain audio tracks, or video, or both. AVIs are primarily known for their association with DivX files encoded as AVIs, which are a common format for video downloads from the Internet. QuickTime Player is a free program for download from Apple that can convert and compress many types of video from one to the other, including MOVs to AVIs. Note that converting video may result in lower quality output, due to each files' compression standards.
Click "File" and select "Open File." Find your MOV in the finder window and open in QuickTime. It will open in its own player window.
Click "File." Click "Export..." to bring up the "Save Exported File As..." window. Select "Movie to AVI" in the "Export:" icon bar.
Click the "Options" tab to change your AVI settings and compressor quality, if needed.
Click "Save" to begin the conversion process. Depending on your file's size, this may take several minutes.