Circuit training routines are one of the fastest ways to get in shape because they are a combination of strength training and cardio in one workout.
If you only have 30 minutes to an hour each day to workout, then circuit training is the most leveraged way to spend your workout time.
You'll do a lot of work in a short amount of time, and your body's metabolism will work at twice its normal rate for the rest of the day.
Circuits are also great because you can do them anytime, anywhere. You can put together a brutal circuit that you can do in your house or even in a tiny room.
You don't need need any equipment or weights, although these things greatly enhance your workouts once you get in better shape.
High intensity interval training is a little different than traditional circuit training. It focuses on maximum effort intervals of cardio. Really demanding but really effective!