Interval Training!! Huh? What Is It Exactly?

If you've ever taken one of my classes, you know I love to use interval training.  But what exactly is interval training?  Why is it good for me?  Why do it?  I found this video with Jillian Michaels, who gives a good explanation and then gives 3 demos of how to incorporate interval training:  http://www.5min.com/Video/Interval-Training-with-Jillian-Michaels-448797082

So... come work out with me... I use this method in all of my classes!  I always try to incoorporate some form of  HIIT (High Intensity Interval Training)!!