Prior to the discovery of Spinning classes, Step Aerobics, and "Low Impact" Aerobics, if a person wanted to take a cardiovascular class at a gym, it was more than likely a "high Impact" Aerobics class. This usually put a lot of stress on the joints over many years of doing it, especially the knees. High Impact Aerobics consists of bouncing on the balls of your feet through out most of the class with a variety of moves to keep your heart rate up, such as jogging, jumping jacks, high knee runs, shuffles, etc...in which both feet come off the floor at the same time. There are modifications a person can do during these classes to avoid the bouncing and the impact.
In "Low Impact" Aerobics, on foot is always on the ground to support the weight of the body. These classes use rhythmic movements and tend to use larger muscle groups to get the heart rate up. BASIC Step Aerobics class is also considered low impact, although many instructors add little hops which are optional. Swimming, walking and bicycling are other ways of getting a good workout without impact of the joints. These are safer activities for people who have injuries, poor balance, and women who are pregnant.
Regardless of the type of activity you choose to do, you will only get out of it as much as you put into it. This would be referred to as the level of "Intensity" at which you perform the exercises. Anyone who has ever ridden a bicycle would know there's a difference between a slow ride on a flat surface and going up a hill. There is a much greater intensity in the latter due to the amount of energy expended to make the muscles in the legs move the bike up a hill. This is an example of a low-impact exercise at a high intensity.
Although walking is a low-impact exercise, jogging and running are both high impact because there is not one foot on the ground at all times. In any given marathon or race there is always someone who comes across the finish line first and someone who finishes last. The difference between the two in the pace or speed at which they are both going. The person running the fastest is using more energy and muscle power to propel him or her forward, as opposed to the person who is jogging and keeping a slower pace. Both of these runners are doing a high impact activity, but one is at a higher intensity than the other.
If you're trying to keep your fitness level up without putting the extra impact on your joints, there are a few ways to do that. Walkers can try adding small hand weights, pick up the pace and try occasional hills to increase the intensity and raise the heart rate. With the exception of the hand weights, bikers can use this same method for getting more out of a workout. If you're a "Cardio Junkie" and love the adrenaline rush you get from sweating and having your heart rate way up, but can't take the impact anymore, try going into a low impact class and keep your center of gravity low forcing yourself to use your leg muscles more. Doing big arm movements will also keep our heart rate up, such as in a kick box class, and will have you walking out of class dripping of sweat and feeling like you had a great workout!