There were several recently discovered vulnerabilities in apple
quicktime software. A lot of people have apple quicktime installed.
Many machines come preloaded with quicktime. Also if you use itunes (or
have an iPod) you most likely have quicktime installed. All someone has
to do to compromise your machine with quicktime installed is to put up a
webpage with an embedded specially crafted quicktime movie. This would
require no intervention from you other then going to a website.