I'm not talking about alternating strength with cardio - that's "circuit" and totally different from true interval training.  Circuits are fine if you are tight on time and can't get to your regular workouts - sort of a maintainence workout if you are already fit.  They are also a great way to gain some overall conditioning when first starting to workout.

But without a doubt, real interval training is such an amazing workout and produces fab results.  If you've never really done it before - check it!

What it is: 

Alternating bouts of high intensity ALL OUT effort followed by rest or recovery.    (Think SPRINTS)

How to do it: 

Gradually warm up for 7 to 10 minutes, then elevate your heart rate up to (if you are starting out) or beyond your anaerobic threshhold (RPE scale 8 is threshhold).  If you are really pushing max VO2, you will be "sucking wind" for sure - nearly impossible to talk and lactic acid in the working muscles means you are there. 

Then recover... walk it out, slow it waaaaay down, let your HR return to the low end or even below your aerobic zone (4 or 5 on RPE scale of 1 to 10)

Starters - use untimed intervals and rest until you feel you can go again.

Timed intervals - 1 to 3 ratio of work/recovery.

If you can go all out for 30 seconds, rest for 90.  With shorter times, you will be able to do more intervals.

If you can go a minute, rest for 3 minutes.  With these longer intervals, you'll end up doing fewer cycles, but are more likely to really hit max VO2.

The hardest part is convincing people to REALLY REST - the harder you rest, the harder you will be able to work in the next round.  If you continue to work in the mid aerobic zone at steady state, that's all you'll ever achieve.

Later you can play with harder ratios of 1 to 2... and you'll see why you should recover.  The shorter the recovery ratio, the more difficult it is to rev up to all out and maintain at the anaerobic level.

Why it's good: 

You get a greater volume of work in a shorter amount of time.

You are increasing your lactate threshhold - this strengthens your cv system and makes you fitter - faster!  This conditions your heart and lungs to respond more safely and efficiently in the event of an emergency... steady state endurance only conditions you to go for a long period of time in your aerobic zone.  Most heart attacks happen in sudden response efforts, so make sure your keeping your heart strong!

Best benefit:

It's the post caloric burn that makes interval training the best cardio fat burning workout ever.  Going anaerobic burns out the stored glycogen in the muscles, leaving only fat stores to fuel your body.  So even at rest, you will continue to burn fat. 

Final thought:

Think of interval as "strength training" for your heart and lungs.  Whenever you train ANY muscle to fatigue, you have to allow the body to strengthen thru the rest and recovery process... so even tho' it's a great way to workout, be smart about it, and rest 48 hours in between. 

Oh... and the standard disclaimer.  Check with your doc to make sure IT is right for you!