The intensity of the exercise that you do determines where the energy will come from in your body. If you do very intense activities -  say short bursts of sprinting - this type of activity will draw its energy in calories from stored carbohydrates. Low intensity long duration exercise will draw its calories from stores of fat  -  these exercises are fat burners.

The low intensity activities like walking, jogging, swimming, biking can be done safely without dealing with injuries - this is an added bonus. It's not fun if you are sore all the time. Also low intensity exercises are easy to do - no straining required. All you have to do is persevere. If you stay indoors on a treadmill or some other trainer - some people watch TV or listen to music etc. so you need not be bored. Other people (like me) just listen to their breathing and their bodies and think of good things while they exercise.

How do you know when you are exercising to burn fat?
This is determined by heart rate and time. Experts agree that if you exercise in the zone where your pulse is between 60 and 70% of its maximum then you should be pulling calories from your fat reserves - in short these limits determine your fat burning zone.

How do I determine my heart rates?
The formula is 220 minus your age; this will give you your maximum heart rate. To determine your zone limits multiply this value by 0.60 to get the low end of the zone and by 0.70 to determine the upper limit.

Pulse Measurement
So if you are 40 years old your maximum heart rate is (220 - 40) or 180 beats per minute (bpm). Now multiply 180 by 0.60 and this yields the low end of the zone or 108 bpm; the upper end of the fat burning zone is (180 X 0.70) or 126 bpm.

You can measure your pulse on your neck or some people like to do it on their wrist. Count it for 6 seconds and multiply by 10 to get your beats per minute.

There are some nice heart rate monitors which you can wear like a watch on your wrist. Some require a strap be placed around your chest which will transmit your pulse to the wrist monitor where you can read it. Many monitors allow you to set the upper and lower limits of your heart rate zone and if the limits are exceeded the monitor will beep warning you.

Also if you workout in a gym many of the treadmills and other trainers have a built in heart rate monitor so you keep track of your pulse while you exercise.

Tips when beginning exercising
At first when you start exercising always check with your heart doctor especially if it has been a long time since you last exercised and/or if you are taking medication. Medication may alter the limits of the fat burning zone.

When you are exercising, you should be able to hold a conversation. If you can't you are most likely moving to fast.

Take several months of going slower than you think you should and let your body develop to handle the more strenuous activity. It's not only your heart that has to develop, but also your legs, back and all soft tissue. Beginners are prone to injury and pain is not fun. Given enough time, you will exercise more strenuously and the fat will melt off - in short, be patient.

Goal
You are striving to stay in the fat burning zone for an hour. If you can do this for 3 or more times a week, and eat a good fat burning diet, you will burn the fat off and keep it off and feel GREAT!

Best of luck!