Swimming is the best low impact exercise around. If you are looking to increase the intensity of your swimming workout, adding hand paddles is one to accomplish that.
Hand paddles come in many shapes and sizes, but most are a variation on the original plastic rectangular design with rubber tubing fasteners. There are some webbed gloves styles as well. Paddles come in different sizes depending on the hand size of the user, as well as desired increase in resistance. The bigger the paddle, the bigger the pull.
Hand paddles increase the amount of resistance you experience with each stroke, so your upper body works more. They also make the flaws of each stroke obvious, and thereby encourage good stroke technique. Hand paddles can, however, tweak shoulders since they can exaggerate bad technique. Remember moderation is key as you add new elements to your swimming fitness routine. Listen to your body, if something hurts - stop.