Not so new anymore- High Intensity Interval Training. Every gym has a class like that, chances are that if you take fitness classes you already had at least one HIIT. If you don't really know what it is all about, or if you took a class and want to know more, keep reading.
HIIT as it suggest by its name is an interval training which means that your exercise time is divided into a few short segments of work and recovery. The Idea behind it is that if you work for a short time (30 seconds to 3 minutes) you will push harder. Which in turn will challenge your cardio-respiratory system (heart and lungs) and you will see improvement over short period of time. You can thin of it as heavy weight workout for you heart and lungs. The more intense your workout segment is, the shorter it is and your body will have to adjust itself to the load (that is the improvement.)
Why should you do it? depending on your goals, if you want to gain muscle mass then you probably need to stick to the weight training, but if you want to improve your cardio-vascular system, incorporating HIIT once a week will benefit you. Even if you are training for a marathon, squeezing in once or even twice a week HIIT session will benefit you. This is not the only form of exercise that you should practice (even though it is fast and intense). When done right, HIIT takes a big toll of your body and you need to let it recover. If you don't allow enough recovery time, the chances to over training increase and THAT is something you want to avoid.
There is a lot of information and research out there about HIIT. It is a great way to reach your fitness goals. Look at it as another tool under your belt. Along with weight lifting, stretching, Yoga, cycling, running, Tai Chi and so on. First decide WHAT is your goal, then choose the right tool for you to use in order to get there, it might be HIIT.