HIIT is a common method used by individuals who are aiming to improve their cardiovascular fitness. It focuses on short bouts of high intensity exercises followed by short rest periods. A simple example of this is sprinting 50m, walking back then going again continuously for 10-15 mins. Make sure you really are pushing yourself and running as fast as you can!
This type of training not only improves your fitness, but it helps burn fat and does so for as long as 12 hours post workout! You can also lose weight without losing muscle at the same time. Long bouts of steady state cardio will eat away at the fat but will also turn to muscle protein as a source of energy.
Here is a great HIIT workout i performed the other day;

20,16,12,8,4 Reps of the following (no rest until complete)

-24” box jumps
-Jumping squats
-Kettlebell Swings