Might as well cut right to the chase. Stephen Seiler's site is not an easy introduction to the topic; it gets into the physiological brass tacks at the heart of the matter.
We carried on quite a conversation about cardio fitness a while back. Once you glean through that, you'll be all set because we discussed: how much cardio? HIIT, High Intensity Interval Cardio Training, Interval Cardio Sample Workouts, Pushing Cars for Cardio Work, Stair Climbing for Lean Legs, Indoor "Spin" Cycle Training, Jogging and Sprint Training, Outdoor Cycling and Cardio on an Empty Stomach?
And we followed that up with a meaty discussion on HIIT (interval training), with examples of intervals, heart rate monitor use and sample training methods, and Byron answers more Tabata questions here.