If you run Windows XP, Windows 2003 (probably not many), or Windows 2000 (again, probably not many), you need to go here to get a fix from Microsoft RIGHT NOW. Just this once, it's OK to use Internet Explorer rather than Firefox; the fix seems to be broken when using Firefox. It should only take a minute.
The problem is that Quicktime-embedded video can trigger silent download and execution of malware (translation: you can be pwned without knowing it simply by watching a video in your browser).
There are exploits in the wild right now. I don't know if they're up on Youtube or other popular sites, but this is urgent.
If you run Vista, or Windows 7, or Linux or Mac, you don't need to worry.
Technical details available here. Also here.