XiphQT (Xiph QuickTime Components) is a set of QuickTime and Macintosh OS X plug-ins. Their role is to provide support for Xiph media compression and container formats.
If you are an end user: installing XiphQT components is all you need to start using Xiph-formatted media files in a variety of applications—for example, listening to Ogg/Vorbis files in QuickTime Player or iTunes. The programming interfaces used in XiphQT allow the provided functionality to be available transparently to all media handling applications using QuickTime or OS X component mechanisms.
If you are a programmer, you have built a Macintosh Core Audio or QuickTime-based application and would like to add Xiph formats support: most of the time there is hardly anything you will need to do to add support.