Apple's Quicktime application has gone through many changes the past year. For those that upgraded from OS X v10.5 and below to v10.6, you can see how much more Quicktime can do. It can now record video (if there's a camera), record audio (if there's a mic), and screen capturing. Now some of these capabilities have been available for previous versions, they were only available if you bought Quicktime Pro, and have now become free. But what Quicktime still does not have is support for certain codec and container support. This is where Perian comes in. Perian is a Quicktime plug-in by The Perian Team that lets Quicktime play file formats that do not instinctively work on it. There are alternatives to playing video files like VLC but it's always nice dealing with just one program. You can get Perian here.