Predominantly a high intensity interval training (HIIT) session is a type of "cardio" session (though I don't like that term as it implies what you see most people doing on cardio equipment; lazily strolling on a treadmill, while holding onto the rails or sitting on a bike reading a book). It is an intense, time effective, weight loss strategy to increase your energy system development, strip fat and do all this in half the time it would take to do conventional cardio. Now, having said that, why why would you want to do HIIT over traditional steady state cardio (long slow continuous- running, jogging, walking etc)? There are many reasons, firstly, it burns more calories (increases fat loss) per capita of work, which means you work for less time to burn more calories, that alone should make it everyones favourite, and preferred form of exercise, period (why would you want to spend all your time in a gym exercising? Why should exercise become a second job?)! The reason intervals are so effective (or at least one of the main mechanisms we are aware of at the moment) is a raise in what is called, EPOC (Excess Post exercise Oxygen Consumption). Now current research by exercise physiologists states that EPOC is not the only factor cause the raising of the metabolism from HIIT, the mechanics by which fat loss occurs over a 24-48hr period after doing them is not quite understood as yet, (it could be due to catecholmine release which increases the effectiveness of fat reducing enzymes) it is only documented as happening. The information regarding HIIT comes from the results accumulated by the worlds leading strength and conditioning coaches as well as being documented by studies.
HIIT actually increases your VO2 Max (basically how fit you are), reduces fat mass, increases vitality (vitality in this case being all positive health benefits (stroke volume, stroke frequency etc) more than steady state cardio, in less time. In fact Michael Boyle, in his HIIT DVD presented evidence that intervals actually retard and reverse the aging process of cells, if that isn't incentive enough to do them I don't know what is. Conversely steady state cardio has been shown to reduce muscle mass (due to increased cortisol production and depleted glycogen) making it harder to burn fat, it causes wear and tear on your joints and you have to do hours and hours of it for it to have a beneficial effect.
So why aren't people the world over doing HIIT over steady state? It's hard, pure and simple! The nature of HIIT is an all out brutal effort, there's no reading a book or watching TV. You need to get into your head that exercise is meant to be difficult, uncomfortable, if you're not asking yourself "why am I doing this?" Then you're not training hard enough! I would also say it's education, the aerobic craze from 20 years ago is still being felt today, it's going to take time to change the cultural paradigm. How do we do that you might ask? By promoting the current science, the current evidence.
Time constraints are an issue or everyone, why would you want an exercise program that consists of several 45min-1hr long running sessions (on top of your work and resistance training), when there is a faster and more effective way? We all have jobs we don't want to make exercise another one! Something to think about maybe?
If you're looking for references I refer you to this blog.