Wet system... Combines gas with the nitrous before it enters your system.
Dry system... Just nitrous sent into your system.
In a 7th gen Celica you have to use a wet kit...
ditto. they say in the 7th gen gts to use a wet system..something about compression when your engine hits lift.
i've heard that if you have a gt, though, that a dry system will do ok.
according to nosnitrous.com, a 1.8L engine should be able to safely have a 30 shot system with hardly any extra wear on your engine (just fyi.)