The general concept of Interval training, is already defined by its name: it indicates the fact that the athlete performs a physical performance for a certain period of time to a greater intensity and heartbeat, interrupted by periods of recovery, before returning to the physical performance.
It contemplates a phase of initial warm-up and final cool-down and this is not anything new or different than other types of training.There are some schools of thought, and then “sub-methods” that use different timing and ratio between performance and recovery, but I think we can basically distinguish between Interval Training (IT) and High Intense Interval Training (HIIT): in the first the activity duration can reach a few minutes, in the second, on the contrary, the moments of physical effort are of such intensity that it can not be sustained for much longer than 60 seconds.
In any case the type of activity and duration depend on the athlete’s physical preparation and on the training program. To further simplify the concept, although today it is used by personal trainers, gyms and is sold as a great way to burn fat, increase metabolism etc.etc.. Just think it is none other than the classic method of training for runners, often used in athletics as well as in preparation for many other sports (football, basketball, rugby).
It is opposed to the traditional Cardio (continuous training): where, to remain in the racing example, you prefer a moderate and continues running to the sprints.
Interval training is a type of training that involves a series of low- to high-intensity exercise workouts interspersed with rest or relief periods.