High intensity cardio can maintain lean muscle, burn more fat then low intensity cardio, builds your endurance, and boosts your metabolism during and after exercise. The types of high intensity exercises you can perform can be any type in which the intensity level can be easily adjusted such as running, walking, and most cardio machines (treadmills, ellipticals, etc.) Basically, it involves interval training which means repetitions of high-speed/intensity work followed by periods of rest or low activity [Source: Wikipedia].
Low intensity cardio has little effect on your cardiovascular system but is good at burning fat. In fact, that is its main job and works well for people who are really out of shape. Low intensity exercises involve any cardio exercise in which you maintain a comfortable pace for the duration of the workout.
When to Use Each Type
Knowing the difference means you can decide when you should incorporate each into your workouts. High intensity cardio can be quite intense since it involves interval training. You’ll also want to eat a sufficient amount of carbs (at least 2 – 3 hours) before a high intensity workout for energy. Many athletes and bodybuilders use high intensity cardio to improve their endurance. I think a couple of times a week of high intensity would work on average but you should seek the advice of a professional fitness trainer to see what works for you.
Low intensity cardio is great for keeping your weight down or initially dropping weight when you are very out of shape. Many cardio machines have a “fat burn” program and 30 minutes or so would probably be sufficient. If you don’t have access to a cardio machine, a nice brisk walk or light jog are good options.