Swimming fins, sometimes called flippers, can be a useful training tool. Like all exercise equipment, however, they have their pros and cons. A consideration of all the aspects involved, as well as the different types of fins, is important for you to decide whether or not you're going to use them in your training.
Arguments For Fins
Swimming fins, depending on the style, can help to build strength in the muscles needed to swimming quickly as well as improve your overall technique. While the exact muscles needed will vary based on your stroke, swimming typically places more demands on your lower legs than most people are used to, and fins can help to strengthen these movements.
As with an aid, it's possible to become overly dependent. If you're used to swimming with fins on all the time, you may be severely disappointed in your performance once you take them off. Fins can also place greater stress on your knees and ankles by increasing the amount of resistance on those relatively sensitive joints.
As mentioned, there are many styles of fins available, but they can generally be divided into two groups: long and short. Short fins create resistance against the water but will force you to use a faster kick. These are most useful for lap swimmers and more serious swimmers who are trying to improve their form. Long fins are better suited for beginners and recreational swimmers. This style fins is especially useful for improving flexibility in your ankles and building endurance.
You should think carefully about what you hope to accomplish by using fins and what sort of swimming you'll be doing before investing in a set of fins. Do some extra research about the design and purpose of the various styles to make sure that your fins help, and not hinder, you.