Wet Suit Sizing Chart
Wetsuits, in a traditional sense, are full body suits made out of neoprene, designed to keep the user warm and protected while in the water. They cover the entire body from the neck to the ankles and wrists, only exposing the head, hands, and feet. They do NOT keep you completely dry. Wetsuits trap a thin layer of water in between the suit and your body, using your body temperature to heat up the water. Other variations of the wetsuit are available, such as a "springsuit" which is much the same as a wetsuit, except it covers less of the body. Different variations include short legs/no arms and long legs/no arms. What type of suit you want is purely preference, as sizing should be the same for all no matter wether it is a full suit or a springsuit.
Just as the title suggests, each wetsuit has its own specific thickness of the neoprene material. Of course a thicker suit will result in more warmth, but it will also lower flexibility. The numbers indicate thickness in millimeters and take the form of X/Y mm or X/Y/Z mm. The first number represents the thickness of the torso. When there is only two numbers, the second number will specify the thickness in the arms and legs. When there are three numbers, the second number will represent the legs, the third is for the arms. Limbs are thinner because they move much more, and keeping your torso warmer is extremely important especially when you are diving in colder waters.