Dry
Nitrous is added at the intake pipe and adds extra fuelling for the pressure created by the nitrous. All dry kits add the pressure to the fuel regulator apart from the Venom kit (pictured below). The Venom kit adds fuel electronically by pulsing the injectors for longer time. Try and keep less then 6" between the nozzle and throttle body
Cost: £600 + £2-4 per lb of nitrous.

Wet
The same idea as dry but the wet bit comes in the form of an extra fuel injector. The fuel rail is tapped for the extra injector - pre-made billet ones are available frm the likes of AEM. The fuel and nitrous are pumped through a Y-shaped nozzle. There's not really much in it between wet and dry.

Direct Port
Direct port kits are the same concept as the wet kit, but there are 4 nozzles, one in each the area between the intake plenium and the head. The minimum N2O shot for a direct port kit is 80-90. All direct port kits must be custom made to fit with plate kits available that bolt between the intake manifold and the head. Plate kits are limited to the amount of cars they are made for - usually Honda.

Engine mods for N2O?

1) Colder plugs around 2 heat ranges cooler.
2) Retard the timing.
3) Purge kit. Purge kits clear out warm nitrous and air bubbles. The kit is also a safety feature, so when you shut the bottle off all the pressure is released out of the line, extending the solenoid life.
4) Fuel pressure regulator and pressure guage so you can keep an eye on things
5) Larger injectors to increase fuel flow.