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.
Safari, playback of ogg from web pages
After installing the XiphQT component, you may hit the problem described in https://trac.xiph.org/ticket/1230 - viz Safari does not launch QuickTime Player when you click a link to an ogg, but instead downloads it.
In order to cause QuickTime Player to be launched and play back ogg files when they are clicked in Safari, the trick is to force QuickTime Player to update its MIME associations, which is not a Safari configuration option, but a QuickTime one. Load QuickTime Player, then navigate menus QuickTime Player -> QuickTime Preferences -> Advanced -> MIME Settings .... open the Audio group, uncheck the two Ogg entries, then press OK. Then reopen MIME Settings ..., recheck the two Ogg entries, press OK again. I found that I needed to toggle them like this in order to get the correct config stored. Now when I click an ogg link in a browser, QuickTime Player is fired up and plays it. Note that I may have hit this problem initially as a result of having installed (and subsequently removed, before installing XiphQT) the VLC plugin.
iTunes 10.5 and XiphQT
iTunes 10.5 is a 64-bit application, which means it will not pick up the XiphQT component, and will not play Ogg Vorbis or FLAC files. Until a 64-bit version of the component is released, you can switch iTunes to run in 32-bit mode by right-clicking on the application icon in Finder, selecting "Get Info", then ticking the "Open in 32-bit mode" checkbox.