If you just want to play AVI footage, either from a digital camera like Canon, or any other AVI videos from other sources, shared by a friend with a PC or downloaded from the internet, you can install the QuickTime component Perian, which helps you play almost any AVI files encoded with different codecs. However, if you want to upload these AVI videos to iTunes for organizing or for transferring to your iPod/iPhone, you need to convert them to an iPod-compatible format like QT MOV first.