High-Intensity interval training - performing repetitions of a series of high-intensity exercise bouts followed by rest - builds up your fitness levels and DECREASES FAT faster than longer aerobic training. Training at this higher intensity level can be done on a treadmill, stationary bike, rowing machine, elliptical or with no machine involved. You can increase your endurance, power and strength in a fraction of the time than the traditional aerobic workout.
No two people are the same when it comes to levels of intensity. One person's rest may feel like high intensity to another person. The key is to start at your own specific level and build up gradually with high enough intensity to burn fat.