Intervals are amazing fitness builders. And if you want to go to the extreme, Tabata intervals can't be equalled.
20 seconds of all out effort, followed by
10 seconds rest
= 1 round
Repeat for a total of 8 rounds.
Try these with burpees, sprints, squats, rope skipping, or any exercise that involves large muscle groups. Tabata Burpees truly suck in ways that cannot be properly articulated with words alone.
For these, you need a decent stopwatch or even better, an interval timer. I use something called a Gym Boss, which is a type of timer that is specifically designed for interval training.