IDVD encoding quality question...?

What is the difference between "high quality" and "professional quality" encoding in iDVD? A "Professional quality" encoding/burning project takes twice as long as a "high quality" project but I really don't see any difference in the finished DVD's playback quality. There must be a big difference of some kind since there's twice as much processing involved in the "professional quality" project. I guess my question boils down to, what exactly is "encoding" and what is better about "professional quality"?