For some time now, two versions of YouTube videos have existed. If you visited YouTube's website, you were served videos through their Flash player. If you visited on a device like an AppleTV, iPhone, or set-top box, you were likely served an H.264 version.
This week, YouTube introduced beta support for an HTML5-compliant player. In short, this is great news for anyone interested in web video. The player loads faster, the videos look better, and your computer/device is less prone to experience Flash-related crashes as a result.
As long as you're using a current (not Internet Explorer 6) browser, you can enable HTML5 in YouTube here.