Well, in general terms for scuba, if it has booties/boots attached to the suit and a funny looking valve right in the center of the chest, it's a drysuit.
Most of the suits you'll see are wetsuits-unless you happen to be diving in a really cold area.
Drysuits require a separate set of underwear as the suit rarely provides adequate insulation. With a wetsuit, the thickness of the suit is the insulation. Wetsuits are generally good to about 55 degrees. Anthing colder and you'd be more comfortable in a drysuit.
The cost difference is substantial. Wetsuits cost anywhere from about $80 or so on up to about $500 for a high quality thick wetsuit. A drysuit could set you back $2000, not including the underwear, which can cost as much as an entire wetsuit.
Surf through the scuba section of E-bay for pictures of all kinds of different wetsuits and drysuits.