HIIT stands for high intensity interval training. There are many different ways to perform a high intensity interval training session the most common type of HIIT workout right now is for time. Interval work to rest times can vary in length and ratio. Usually between 10-40 seconds and the ratios are between 1:1 of 3:1
Circuit refer to performing exercises in succession with little to no rest between exercises. Exercises can be the same muscle group or more commonly varying muscle groups creating an interval style workout. For example alternating ab exercises with leg exercises, so that you rest the legs every time you work the abs and vice versa. Circuit exercises can be assigned repetitions, time. distance or a combination of those increments.