Wet
Suits 
Wet
Suits
My theory on wet (or dry) suits is that if you dive, you need one (or more). The reason is to stay warm; to stay comfortable so you can better enjoy your dives; especially repetitive dives. Which one you get depends on where you dive, and of course, the temperature. Owning one or more can insure you have the right suit for the right dive.
The basic dive suits are Dive Skins, Shorties (Spring Suits), 3mm full, 5-7mm full, 7-9mm Farmer John (2 piece), and the Dry Suit.
Dry suits are made to keep you completely dry, and necessary for the coldest waters and for prolonged or extended time in cooler waters. Technical divers tend to wear dry suits because of their extended dives and decompressions.
The Farm John (or Jane) is a step in, thick, 2 piece suit that is a good universal suit, since you can wear only half (top or bottom) in warmer waters, but is usually used in colder waters.
The 5-7mm, once piece suits are for colder waters. I use a 7mm for my California and Florida Spring diving, where the temperature can be as low as 50º.
A one piece 3mm suit (can be 2mm or even 4mm) is my best recommendation as an all around suit for most conditions. It isn't overkill in the tropics, and good enough for short 70º dives.
The Shortie or Spring Suit is great for snorkeling in warmer water, jet skiing or even to wear under a 3mm suit in colder waters.
Dive Skins are not made to keep you warm. The are made to keep the elements like sun, coral scrapes and stings from harming you. They can also be used under a wet suit to make putting them on easier.
Most wet suits are made from neoprene which is a semi porous material. This means it contains millions of tiny air pockets or bubbles that form an insulation layer between you and the cold. A thin layer of water will enter your suit, but this is quickly warmed to your body temperature and as long as your suit is not overly loose, provide you with a warm and comfortable dive in waters cooler than your body core (98.6º).
Some shortcomings of wet suits are:
a) They can cause you to become overheated in warmer
climates
b) They can chill you if you don't remove them (at least the top part)
between dives
c) They can dramatically increase your buoyancy (this means more weight to
carry on your belt)
d) A loose fitting suit is practically useless as a defense against the cold
since it allows water to circulate
