Fin socks can really come in handy when you are using swim fins or training fins. They act as a cushion between the rubber and your feet. They can be useful for a few reasons. First, they can help if your fins are hurting your feet.
Second, if you bought a pair of fins and they are a too big, they will help tighten the fit. Lastly, they can really make a difference when the water is very cold. When the water is very cold, the fin bootie is a better option.
Some fin socks are made out of nylon/lycra and are slightly different than socks made from neoprene. These socks are lightweight and thin. They have less cushion and do not keep you as warm but they are more flexible and maneuverable. They are mainly used to aid any rubbing or hurting of the swim fins. Many times the friction from the rubber can hurt your feet.
This is simply the material most fin socks are made out of. It is the same material wetsuits are made from and it has a thick, soft, and cushiony exterior. They are always low cut on the ankle and are sometimes exposed at the heel.
Be advised though these work slightly for keeping your feet warm; there are a lot of exposure points for water to seep in. They add relief if fins are hurting your feet by forming an extra level of support. Add cushion if your fins are a bit too big.
Swim fin booties are a heavy duty version of fin socks. The entire foot is covered and the material raises several inches above the ankle like a regular sock would. In addition, they are thicker in material than normal fin socks.
You may need a slightly bigger fin because they are very thick and full. The main purpose is for very cold water, where your feet would otherwise go numb in the water. For my full guide on swim fin booties click here.