HIIT stands for “High Intensity Interval Training”--a style of exercise where very intense rounds (or “intervals) are alternated between short rest periods. Because the effort isn’t continuous, HIIT workouts are more intense than regular cardio. However, unlike resistance training (which is also intermittent with short rest periods), HIIT workouts tax the cardiovascular system in a way similar to cardio.
Many proponents of HIIT will argue that interval training is better than regular cardio. Instead of going for a 45 minute run, a HIIT workout might entail running 100 meter sprints several times. This shortens the workout while still getting in a lot of intense training. Because the training is more intense, and the workouts are shorter, proponents will claim that HIIT preserves or even builds muscle better than standard cardio.
HIIT is sort of a middle-ground between cardio and resistance training. Resistance training preserves and builds muscle better than HIIT, and cardio burns more energy than HIIT, making interval training a sort of “jack of all trades, master of none.”
Ultimately, the choice between HIIT and cardio is a matter of preference. If you’re okay with longer workouts and just want to burn as many calories as you can, going for a long, slow run will be your best option. However, if you can only do short workouts and don’t mind suffering through very intense training, HIIT could be for you, and might even be more fun.