High Intensity Interval Training is short intreveals followed by a short rest.
Ex: Jumping Jacks for 45 seconds, 15 second rest, Burpees 45 seconds, 15 second rest.
According to research HIIT, because of the intensity, uses glycogen or carbohydrates as its main source of fuel. HIIT adds stress to the body and is harder to finish.
Low Intensity Steady State, involves a steady pace for a period of time.
Ex. 12-3-30 on the treadmill, the stair master, jogging, walking, etc.
LISS forces the body to use fat as its main source of fuel for the workout. LISS takes longer to finish and is easier on the body.
Which one you choose to do depends on your goals and how much available time you have.
To get benefits from both I recommend a 2:1 ratio of HIIT and LISS a week, doing 2 HIIT workouts and 1 LISS.