From Apple:
Plug-In Helper is an application which allows you to:
Associate URLs with QuickTime movies and video tracks.
Store QuickTime Plug-In settings inside a QuickTime movie.
The QuickTime Plug-In uses the information you add when the movies are played back over the web. Examples of the kinds of information you can add to a movie include plug-in playback control settings (whether to start playing the movie autmatically, whether to show the movie controller or not), and URLs to jump to when the user clicks on the movie or a particular track of the movie.
