Answer:
The various swimming organizations put out rulebooks which define the proper actions that a given stroke is allowed to make (e.g. how to swim butterfly). When swimming in an event specifying a specific stroke, the swimmer must abide by these rules defining the allowable movements.
In addition, there are general rules covering the construction of the pool, such as the minimum lane width, lighting, temperature of the water, height of the gutters and starting blocks, etc. Swimmers much also follow rules around non-interference with any other swimmer (e.g. keep all your body parts inside your own lane), and rules about proper transitions - e.g. how the start is handled, and how relay races are to be conducted.
For specifics, you will have to consult the published rulebook for the various swimming organizations.