High Intensity Interval Training. It's a short form of training (no more than 20 minutes) that focuses on short sprint intervals (or a similar high effort cardiovascular activity) followed by a jog (or a similar medium effort cardiovascular activity). For example:
4 minute warmup jog
30 second sprint - 30 second jog (this is one "rep", so you'd want to do that maybe three-four more times when beginning a HIIT program)
4 minute cooldown jog
As time goes on you could add more reps in to your HIIT, or change it up to something like 20 second sprint / 10 second jog reps. There's plenty of different methods you can try.
