Paddles are used for improving technique (with some added resistance) or for building upper body strength through added resistance.
Technical paddles such as the Maru, Speedo tech or Finis Agility paddle are great for those looking to use this training aid for improved stroke quality. Swimmers looking to increase their resistance workout should use a larger flatter paddle to increase the surface area of the hand and increase the volume of water to be moved during the pull phase of their stroke. The Speedo Power and Biofuse paddles serve this purpose as do the TYR Catalyst and Arena Vortex paddles.
In addition to the basic paddles more specialist paddles exist to improve strokes such as the Finis Freestyler paddle. Finger paddles are used for getting a better feel for the water during skulling drills and are becoming increasingly used by junior swimmers instead of hand paddles.