First, it is essential that you understand that if you are a male older than 45 or female older than 50 years old and if you have diabetes, heart disease, insulin resistance or hypertension you should get medical clearance prior to starting any Moderate or Vigorous exercise program.

Moderate exercise is defined as exercise that increases your heart rate to 60-70% of your maximal heart rate. Your maximal heart rate can be calculated easily by subtracting your age from 220. Moderate exercise is often defined, also, as the range that causes the greatest fat burning. If you are not sure how to take your pulse, you can determine your level of exercise by the exercise intensity that causes you to break a sweat during the activity, however, not so intense that you are breathless. You should break a sweat, yet still be able to carry on a conversation. If you can't carry on a conversation you are above the moderate range.

Vigorous exercise is 70-80% of your maximal heart rate. This is also the range were you break a sweat and have difficulty finishing a sentance in a conversation due to breathlessness. Exercise in this range can put someone at risk for a heart attack if they have coronary artery disease. Therefore, it is important to ensure that you have no risk for heart diease prior to starting an exercise program that incorporates exercise at this level. (http://www.uptodate.com/online/content/topic.do?topicKey=genr_med/6983&selectedTitle=2%7E150&source=search_result)

I hope this answers your question.