Here is a simple explanation of each of the four main swimming strokes.
In freestyle swimming events, as the name implies, the stroke used is not limited by any particular technique. Swimmers in freestyle events are free to use any stroke they wish, it is just that the stroke that is commonly called freestyle (which is also known as the front crawl or Australian crawl), is the fastest swimming stroke. The only limits is that only 15 meters can be swum underwater (from the start and from each turn), otherwise some body part must always be above the water. Some believe that the front crawl stroke was developed by Richard Cavill, an Australian who first combined the overarm stroke with the up and down kicking motion.
Swimmers of the breast stroke must follow strict rules when performing the stroke. Their shoulders must be kept in line with the water, arm and leg movements must be pushed forward together, and brought back under the surface of the water. At the turn and finish, both hands must touch the wall together. At the start and first stroke and kick after a turn, the swimmers are allowed one arm stroke and one leg kick. At all other times the swimmer's head must be kept above the surface of the water. No dolphin, scissors or flutter kicks are allowed, nor tumble turns.
The butterfly stroke evolved from experiments with the breaststroke. This technique may have been invented by the German E. Rademacher in 1926, and improved upon by American Henry Meyer and accepted in competition in 1933. However, it took many controversial years up to 1953 before the butterfly stroke was officially recognized and included in the Olympic Games (in 1956). When swimming the butterfly stroke, the swimmers must keep their shoulders in line with the surface of the water, and make arm and leg movements together. They also must not swim underwater, except for the first stroke after the start and each turn. Only 15 meters are allowed underwater at the start and after each turn, and the wall must be touched with both hands.
When swimming the backstroke, the swimmers remain on their back. This technique was first swum with a frog kick (like the breaststroke) then the up and down from that is used now. Similar to the freestyle, only 15 meters can be spent underwater from the start or from each turn. In 1991, the rules were changed so that when turning, the swimmers did not have to touch the wall with their hand, enabling them to do a much faster turn.
In the individual medley races, the order of strokes is butterfly, backstroke, breaststroke, and freestyle. In the medley relay, the order is backstroke, breaststroke, butterfly, and freestyle, with the first swimmer starting in the water.