Swimming describes any method by which humans (or other living creatures) move themselves through water, apart from walking on the bottom.
Swimming is a good form of exercise. Since the body weight is supported by the water, practically all the effort goes into forward propulsion. And since propulsion is hindered much more by water than by air, the efficiency and effectiveness of the swimming movements are much more dependent on strength and technique than, say, in running. Swimming is therefore an exercise that gives both cardiovascular and strength benefits, particularly to the arms, legs, upper torso and the abdominal and back core muscles, while at the same time fosters attention to technical detail.
Swimming is also a good way to relax. Many recreational swimmers prefer a style that keeps their head out of the water and uses an underwater arm recovery – in practice that means breaststroke or some variant of sidestroke. Sidestrokes are also the way most cultures around the world developed swimming techniques before the modern era of competitive swimming.
Furthermore, because the density of the human body is approximately similar to that of water, and thus the body is supported by the water, less stress is placed on joints and bones. The resistance against movement depends heavily on the speed of the movement, allowing the fine tuning of the exercise according to one's ability. For these reasons swimming and other exercises in water is frequently used in rehabilitation after injuries, or for the disabled, or as a complementary exercise for athletes of other disciplines.
Questions that should have this tag deal with, for example, swim techniques (stroke, breathing), injuries, dry-land exercises, open water swimming, swimming in combination/relation with other physical activities, or work-out routines for swimming.
See for example these questions to learn more about Swimming at this site: