For example: I live a stone's throw from where venerable Web 1.0 company iPIX used to exist. iPIX, for those who remember, pioneered a way to record and play back immersive video just like QTVR. Initially their tech was only available via RealPlayer, which, at the time, had the most open architecture. Eventually they made a QuickTime version, as did about a dozen others. Recently I tried to play a training video, and I needed the TechSmith codec. I had to download the component, and everything was viewable. Perhaps the best known and widely used component is the MPEG-2 codec, which is needed for DVD authoring.
Components can be codecs, but they don't have to be. The categories of QT components can be found here, although it's a bit technical. The hard part, honestly, is figuring out which components to install. But since there are only 11 on Apple's QT components page, I suggest installing them all and enjoy yourself. And shame on you lazy devs out there who have yet to implement a Mac version! In some cases, the open source community has taken up the gauntlet. Witness XiphQT, an effort to bring Ogg support to QuickTime. A lot of the furor over components has been laid to rest, however, as we see more specialization. For instance, video via Flash is now a popular method of putting videos on the web. This obviates the need for 3 different format buttons (remember those, along with hi and lo bandwidth versions?). Also, specialized content is being embedded in everything from Adobe's PDF format to specialized readers anyway, making the discussion less relevant every day. One special note of caution: the Apple page lists PowerPC components. Unfortunately, you have to check each component on their home site to find Universal Binary editions. The PPC versions will NOT work on an Intel Mac.