Scuba wetsuits keep divers comfortably warm by trapping a layer of water against the skin.  Your body heat warms it then the wrist and neck seals keep that warmed water in.  It acts as the insulator to protect you from the cold water outside the suit. 

Drysuits use air and long underwear for insulation.  Ice divers use drysuits year 'round and vary the underwear bulk to temperatures expected.  Hypothermia can occcur 30 times faster underwater than in air! So, dress for the coldest conditions you'll be diving.

Wets offer a variety of styles. Use depends on the water temperature and frequency of your activities.  For temps under 80 degrees F., you can wear a torso covering Spring Suit or Shorty (short sleeves and legs) that half zips up the front.  For waters in the 70s or multiple dives per day,  use a one piece Fullsuit which covers the arms and legs completely that has either back or front zip closures. Two piece suits consisting of a Farmer John (Jane) overall-like base with a short-legged (step-in) jacket make maximum thckness insulation for colder water dives. 

Materials vary from neutrally bouyant fabric diveskins (about 1/16th inch thickness) for sunburn protection Snorkeling and tropical diving, to 1/8th inch (2-3 mm) neoprene rubber for low bulk insulation, and 1/4th inch (5 or 7mm) coverages. Some technical improvement in heat retention is claimed for titanium oxides added to the neoprene. 

Generally, thicker is warmer.  The tradeoff is that you need more lead to offset increased bouyancy of the thicker neoprene wetsuits.  And thicker may mean less flexibility, too.  Try layering two or more thinner pieces rather than wearing a single thickness suit.

Men's suits are cut larger in the chest; women's have more hip room and tailoring.   Technical fabrics bonded inside the neoprene suit make tight fits easier on and off.  The grip must be taught but not choking to retain maximum heat.  There should not be gathering in the small of the back or under arms from a too loose suit. Water sloshing in and out of swim, ski, or surf suits offer no value to divers.  Shop wisely! 

Since most heat loss occurrs from the head and kidney areas, it is important to preserve heat by insulating those areas first.  Hoods either stretch on or have zipper closures. Their bib goes inside the wetsuit collar.  No-bib styles are for drysuits.  Hoods also come attached to Vests for intelligent layering.

Personally, I use a thinner fullsuit on every dive and add a hooded vest under the suit for cooler weather. 
Check with your local dive stores for their advise or Ask The Seller in MY Auctions on eBay.