AVI is Microsoft’s video format. AVI is popular because it can take advantage of newer, and better compression schemes as they are developed. The data inside an AVI file can be compressed in many different ways. This is possible because AVI is designed to work with software called “codecs.” A codec decodes the compressed data into a movie. So, for example, if you have an AVI movie that was compressed with DIVX, Windows Media Player will use the DIVX codec to decompress the movie as it plays.
AVI’s flexibility is also a drawback because if you download an AVI file, you often can’t tell in advance if you will be able to play it. You may have to spend time hunting down and installing the proper codec.
QuickTime can play AVI’s on the Macintosh, but you still need the proper codecs. Mac users should keep in mind that the people who create AVI’s do so on Windows computers, and that sometimes the movies won’t play properly on the Mac.
Another option for the Mac is the free VLC media player. At the time of this writing, Apple was not permitting VLC in the App Store. However, you can download it from the VideoLAN website. VLC has many codecs integrated, so chances are that it can play your AVIs right out of the box. VLC is free, open-source software, and highly recommended. Note: there are a bunch of scam sites out there charging money for VLC. Just ignore them. The program is definitely free.