HIIT, or High Intensity Interval Training, is becoming one of the most popular forms of working out because it is insanely effective in speeding up the fat burning process. What's even better is that the entire duration of the exercise should only last between 15 and 20 minutes, meaning it can easily be incorporated into busy schedules. Basically, if you're looking to build muscle and lose fat in under 20 minutes a day, HIIT is your perfect exercise.
How does it work? HIIT alternates the intensity of the workout between moderate and intense, making your body perform to a level it simply cannot reach when working out for longer periods of time. It's basically a "give it your all for 15 minutes" sort of workout, rather than a "run on the treadmill at half speed until you decide you're done" one. The high intensity revs up your metabolism, allowing you to burn fat all day long, even when you aren't working out!
Here's a good beginner's HIIT program. Try this out and see how you feel! Once you see results, you won't want to go back.