You may have used them or seen others use them at the pool – big paddles either strapped to swimmers’ hands or held in place with a hole for your thumb.
Usually they are used with a buoy held between your legs, just above the knee – to keep your hips up so you can focus only on the pull part of the stroke without kicking.
I think they are generally accepted as part of training for swimming, though I know there is also a school of thought not to break up training with using tools such as paddles.
Personally I love to use them – I go SO much quicker with them and love the feeling of gliding through the water. It is almost relaxing and effortless (yes, this may mean that I have a really sucky kick! or a variety of other things).
Some of the reasons touted for using them include: building strength (because of the added drag to pull through the water), to spice up swim training so you are not doing the same thing, and to help with technique. A coach once told me that using them helps correct your stroke without paying much attention because of how you have to hold your hands and arms. This seems true in my own training, and it also seems that I can more easily focus on my stroke.
I have these paddles – the medium size. I love them, though I’m not sure there is a huge difference, and most pools have paddles and buoys (for your legs) that you can use.