I recently bought this neat little Reebok FitWatch that measures your heart rate ... and with the data of my age, sex and current weight ... help also to determine my caloric expenditure. Below is a breakdown of the different zones, and supporting references.
*Your maximum heart rate is about 220 minus your age. The figures below are averages, to only be used as general guidelines.
Healthy Heart Zone (Warm up) 50 - 60% of maximum heart rate:
The easiest zone and probably the best zone for people just starting a fitness program. It can also be used as a warm up for more serious walkers. This zone has been shown to help decrease body fat, blood pressure and cholesterol. It also decreases the risk of degenerative diseases and has a low risk of injury. 85% of calories burned in this zone are fats!
Fitness Zone (Fat Burning) 60 - 70% of maximum heart rate: This zone provides the same benefits as the healthy heart zone, but is more intense and burns more total calories. The percent of fat calories is still 85%.
Aerobic Zone (Endurance Training) 70 - 80% of maximum heart rate: The aerobic zone will improve your cardiovascular and respiratory system AND increase the size and strength of your heart. This is the preferred zone if you are training for an endurance event. More calories are burned with 50% from fat.
Anaerobic Zone (Performance Training) 80 - 90% of maximum heart rate: Benefits of this zone include an improved VO2 maximum (the highest amount of oxygen one can consume during exercise) and thus an improved cardiorespiratory system, and a higher lactate tolerance ability which means your endurance will improve and you'll be able to fight fatigue better. This is a high intensity zone burning more calories, 15 % from fat.
Red Line (Maximum Effort) 90 - 100% of maximum heart rate: Although this zone burns the highest number of calories, it is very intense. Most people can only stay in this zone for short periods. You should only train in this zone if you are in very good shape and have been cleared by a physician to do so.
MayoClinic's: THR Calculator
Article: AHA: Target Heart Rates