Jogging
Jogging is a form of trotting or running at a slow or leisurely pace. The main intention is to increase fitness with less stress than actually running, instead of competition.
Definition
The definition of jogging as compared with running is not standard. Dr. George Sheehan, a running expert, is quoted to have said "the difference between a jogger and a runner is an entry blank". Others are usually more specific, defining jogging as running slower than 6mph (10 minute per mile pace).
A jogger is also a derogatory term for competitive runner who lack speeds, endurance, form etc.
Exercise
Jogging is a "high-impact" exercise that places strain on the body, notably the joints of the knee. This is actually one of the basic reasons for doing the exercise, as the impact drives growth processes in the areas of the body stressed by that impact. It is no more harmful than other forms of impact activities like jumping or skipping though, albeit with one leg.
Some people drop jogging in order to take up "lower-impact" exercises such as stair climbing, swimming, cycling or walking.
Jogging is often used by serious runners as a means of active recovery during interval training. The runner who may just have completed a fast 400 metre repetition at a sub-5-minute mile pace, may drop to an 8-minute mile pace for a recovery lap. The jog might be carried out in much poorer, looser form whose purpose is to "shake out" the body and maintain circulation to eliminate from the muscles metabolic waste products produced during the bout of hard work.
Like other types of aerobic exercise, jogging is an excellent means of improving cardiovascular health, bone density and physical fitness.
Jogging as a sport
Because jogging isn't a well-defined term and doesn't aim at achieving any specifically identifiable goal, jogging cannot be classified as a competitive sport. There isn't any clear set of rules by which competitors could be disqualified for cheating by transitioning from jogging to running, other than the general observation that they are running with too good a form, and trying to win by moving too quickly.
Poor form
Jogging is also characterized by what might unkindly be termed 'poor form'. Joggers, or runners who are jogging, sometimes demonstrate a hunched posture, carry their arms too high, and leap excessively high into the air and land heavily on the heel. Such form wastes energy and exacerbates the impact of the exercise. An elite long-distance runner can move three times as fast as a jogger, yet experience much less impact due to a smooth form that minimizes vertical motion, and which doesn't exhibit the heavy rear-foot landing during the footstrike.