Freestyle can technically be any stroke, but front crawl is most common.
Front crawl, commonly referred to as freestyle, is swum with an alternating arm stroke, flutter kick and breathing to the side. It is the most common swimming stroke and often the first one novice swimmers learn. Front crawl is the fastest stroke for most swimmers, and the stroke of choice in the swimming leg of triathlons.
Front crawl uses the flutter kick -- a quick, alternating leg, up-and-down kicking style. Flutter kick is done at the surface of the water, with feet just breaking the water's surface as they kick. Flutter kick, executed properly, helps the swimmer maintain a proper bodyline in the water, reducing drag and helping to propel the swimmer forward through the water. This kick starts at the hips -- not the knees -- and is powered by the upper leg and core muscles. Knees should be straight but not locked, ankles should be flexible and toes should be pointed.
In front crawl, the muscles of your shoulders and back power your arms as they alternately reach forward, first "catching" the water with the hand and then pushing it back toward the feet. This pull, helped by the kick, propels the swimmer from one end of the pool to the other. Fingers should be together, but relaxed, forming a paddle of the hand. Elbows are high -- always higher than the hands -- and the elbow straightens as the arm reaches forward to enter the water, and again as the arm reaches back during the pull.
Breathing and Bodyline
Rotary breathing, or breathing to the side, is used for front crawl. The stroke is swum with your face in the water, slowly exhaling continuously and looking down until another breath is needed. To get a breath, your head turns toward the arm that is pushing the water back toward your feet, returning to the water as that arm reaches forward. Bilateral breathing -- alternately breathing to both sides -- is a good habit to develop because it can help increase your oxygen intake, according to USA Swimming, while also improving your bodyline to reduce drag.
To build a stronger pull, swimmers can use a pull buoy -- a foam float held between the thighs -- to support the legs, eliminating the need to kick. This allows the swimmer to focus energy and attention on proper pulling technique. Hand paddles or swim gloves can be worn to add resistance for muscle toning and build strength. To work on the flutter kick, swimmers can use a kick board to allow the arms to rest while the legs get a good workout. Fins can be added to kicking drills for added resistance and to help the swimmer develop proper kicking technique.