A moderate exercise intensity means that your heart rate is between 60 and 80 percent of your maximum heart rate. Less than that and your arent taxing the cardiovascular system enough to really see improvement. More than that and you simple cant keep up with your bodys demand for oxygen and you run out of gas. (for a more detailed discussion of these energy systems, read How to Design an Exercise Program).
To determine your heart rate range, subtract your age from 220, then multiply the result by 0.6 to determine the low and by 0.8 to find the high end of your heart rate range. While exercising, you can stop and check your pulse every so often to determine if you are in this range then adjust the intensity of your exercise as needed.
If your goals extend beyond simple health improvement, you will need to adjust your exercise time, type and intensity accordingly. By gradually increasing the duration and intensity of your aerobic sessions, you will continue to improve your cardiovascular efficiency. By focusing on sport-specific training, you will develop skills for a particular event. Weight training can also be added and for a well-rounded program.
Once you reach a base level of fitness, other goals are often in sight. Weight loss, sports training, fun runs, marathons, century bike ride and other goals such as achieving a personal best. The amount of exercise and the type of training required to achieve these goals are very specific to your event and your current fitness, but there are many tried-and-true training programs for every possible goal.
For more about Training Techniques and Tips, check out the following resources:

