Training zones are a classification system that divides exercise intensity into 5 to 6 distinct levels. Each zone corresponds to specific physiological adaptations. The most common model uses 5 zones based on heart rate (Karvonen method) or pace (% of MAS). Polarized training recommends spending 80% of time in zones 1-2 (easy) and 20% in zones 4-5 (intense), avoiding the 'gray' zone 3.