In a "wet" nitrous system, extra fuel is injected into the intake plenum via a fogger nozzle. In a dry system, only the stock fuel injectors are used. "Wet" systems are used for NOS kits with higher HP levels. Typical street kits, like the RM racing setup for the NSX, are of the "dry" type.
If you check out a Summit or Jeg's catalog, you'll see that the racing kits that add 150hp or more are mostly of the "fogger" (wet) type.