If you do water sports in cold water such as surfing, you know that you need to wear a wetsuit to keep you warm while you are in the water. A wetsuit serves to keep you warm by insulating your body against the cold water around you. Contrary to what some people may think, a wetsuit does not keep you dry on the inside. A wetsuit will actually let in a small amount of water that it keeps near your body. Your body will warm this water to the same temperature as your body temperature and the wetsuit will keep this water tight against your body. Wetsuits are made from neoprene, which is a synthetic rubber that is a very poor conductor of heat. Heat remains trapped near your body which will keep you warm.
It is very important that a wetsuit fits you well. If the wetsuit is too big for you, gaps and air pockets will form around your body. Water can then freely exchange from the inside of your wetsuit to the cold water around you. The warm water that is exchanged for cold water is your body heat escaping and over time you will become very cold. Cold water exchange can be very unpleasant as anyone that has ever worn an ill-fitting wetsuit will testify as you feel cold water rush down your back or through your torso. Thus it is very important for a wetsuit to fit very snugly to not allow water exchange.
The thicker the neoprene is on the wetsuit, the better it insulates you and therefore better to keep you warmer. However the thicker the neoprene, the more restrictive it is to your movement. Therefore many wetsuits are constructed with two separate widths of neoprene. A wetsuit that is referred to as a 3/2 is made of 3 mm neoprene around the torso because the torso requires less movement an 2 mm of neoprene around the arms and shoulder because those areas need more range of motion.
Neoprene quality can greatly affect your range of motion in a wetsuit. Higher quality neoprene will be more stretchy than cheaper low quality neoprenes. The quality of the neoprene will make all the difference in the world in your experience.
However neoprene quality will be reflected in the price of the wetsuit. Premium wetsuits made of high quality neoprenes such as Patagonia wetsuits and the upper line of Oneill wetsuits are constructed of very high quality neoprenes that you can feel in your fingertips.
Take some time to try on many different wetsuits to find the best wetsuit for you. The wetsuit needs to fit you snugly but also provide you with a good range of motion. Having the wrong wetsuit can totally undermine the joy of your water activity.