For a while now, digital cameras have been able to record movies along with just taking photographs. Camera vendors use different video codecs to compress and save the video you take. Common codecs include one of Apple’s quicktime codecs (usually saved as a .mov file) and a variety of codecs with the .avi extension. AVI files are commonly associated with Windows Media Player. To make the situation more confusing, movies encoded with the popular DivX codec might be saved by your camera as a. .avi file. Unfortunately, if you’re a Mac owner, if your .avi movies won’t playback, its likely that you need to install DivX.
The X Lab has a whole page on why some .avi files may not play on Max OS X, and suggestiong for how to play them.
To further confuse the issue, many DivX-encoded files carry the .avi extension. QuickTime does not include native DivX® support, even though QuickTime 6 and later support the ISO Standard MPEG-4 media compression format and DivX is based on the MPEG-4 standard.
DivX also makes their codec available as a free download, and it can be used from any application with QuickTime exporting capabilities.
Play DivX videos in QuickTime, Front Row and many other 3rd party media players
If installing the codec solves this problem for you, please leave a comment to let me know!