Freestyle is an unregulated swimming style used in swimming competitions

Swimming (sport)

Swimming is a sport governed by the Fédération Internationale de Natation .-History: Competitive swimming in Europe began around 1800 BCE, mostly in the form of the freestyle. In 1873 Steve Bowyer introduced the trudgen to Western swimming competitions, after copying the front crawl used by Native...

 according to the rules of FINA

International Swimming Federation

Fédération Internationale de Natation is the International Federation recognized by the International Olympic Committee for administering international competition in the aquatic sports...

. The front crawl

Front crawl

The front crawl, forward crawl, or freestyle is a swimming stroke usually regarded as the fastest of the four front primary strokes. As such, the front crawl stroke is nearly universally used during a freestyle swimming competition, hence the synonymously used term "freestyle". It is one of two...

 stroke is almost universally used during a freestyle race, as this style is generally the fastest. As such the term freestyle is often used as a synonym for the front crawl.
Competitors in freestyle swimming can use any of the unregulated strokes such as front crawl

Front crawl

The front crawl, forward crawl, or freestyle is a swimming stroke usually regarded as the fastest of the four front primary strokes. As such, the front crawl stroke is nearly universally used during a freestyle swimming competition, hence the synonymously used term "freestyle". It is one of two...

, dog paddle, sidestroke

Sidestroke

The sidestroke is a swimming stroke, so named because the swimmer lies on one side and it is helpful as a lifesaving technique and is often used for long-distance swimming. The sidestroke allows the swimmer increased endurance because, instead of working both arms and legs simultaneously in the...

, etc.