In everyday fitness talk, “cardio” usually means steady-state aerobic exercise: you keep a moderate, sustainable intensity for a longer period (often about 30–90 minutes), such as jogging, cycling, or swimming. High-intensity interval training (HIIT) instead alternates short bursts of near-maximal effort with recovery, so sessions are typically shorter but harder on the body and need more recovery between workouts.
Both approaches can improve cardiorespiratory fitness and support calorie burn; HIIT is often more time-efficient, while steady-state cardio is generally easier to sustain and recover from. Many programs combine both. Source: American Council on Exercise (ACE).