The four strokes
The four competitive swimming strokes are freestyle, backstroke, breaststroke and butterfly. The combination of all four strokes is called individual medley.
In freestyle events, the competitor may swim any stroke. The stroke most commonly used is sometimes called the crawl, which is characterized by the alternate stroking of the arms over the surface of the water surface and an alternating (up-and-down) flutter kick.
Backstroke consists of an alternating motion of the arms with a flutter kick while on the back. On turns, swimmers may rotate to the stomach and perform a flip turn and some part of the swimmer must touch the wall. The swimmer must finish on the back.
The breaststroke requires simultaneous movements of the arms on the same horizontal plane. The hands are pressed out from in front of the breast in a heart shaped pattern and recovered under or on the surface of the water. The kick is a simultaneous somewhat circular motion similar to the action of a frog. On turns and at the finish, the swimmer must touch the wall with both hands simultaneously at, above or below the water surface.
Some consider the butterfly to be the most beautiful of the strokes. It features a simultaneous recovery of the arms over the water combined with an undulating dolphin kick. In the kick, the swimmer must keep both legs together and may not flutter, scissors or use the breaststroke kick. Both hands must touch the wall simultaneously on the turns and the finish.
The individual medley, commonly referred to as the I.M., features all four strokes. In the IM, the swimmer begins with the butterfly, then changes after one-fourth of the race to backstroke, then breaststroke and finally freestyle.
Source: USA Swimming