Cardio is basically eveyrthing that gets you breathing hard and your heart rate up. Intense cardio is anything that will get your heart rate and breathing really up there.
An equal opponent in man to man (woman to woman) combat of course, nothing will get your heartrate up higher and longer as when someone is trying to destroy you. That or sex ;)
Intense exercise increases endurance and challenges your body more than low-intensity exercise. Also, high intensity exercise like weight lifting increases muscle mass, which is very metabolically...