Wet system? Dry system?

What do these mean? I don't get it!

Depends on what you talk about. Most common to talk about wet and dry systems are probably clutches, nitrous injection and lubrication systems.

If your referring to N20 then...

Wet System - When nitrus is combined with fuel is the throttle body before it is taken in by the engine

Dry System - When nitrus is injected in itself through the air intake and fuel is pumped in seperetly

The only real diffrence between how you do it depends on how you tune it. You have to be sure to get the right combonation of Fuel/Nitrus otherwise it could burn too hot or be more prone to detonation or other issues.

Yeah I was referring to N2O.

So a wet system is when N2o is injected with the fuel ? A dry system is when N2o replaces air ?

Yeah I was referring to N2O.

So a wet system is when N2o is injected with the fuel ? A dry system is when N2o replaces air ?
No, a dry system adds N2O to the intake air and the extra fuel needed is injected separatly, for example by increasing the fuel pressure in an fuel injected car.

A dry nitrous sytems is when you are spraying nitrous into the car but no addition fuel lines have been added to spray in more gas. You will get the extra gas through overdriving you injectors and pump.
A wet system will spray nitrous and fuel through an additional line usually through the intake unless it is a fogger system.

A dry system is safer to run for the beginner, because it's less involved.
A wet system is more advanced and caution should be used in a wet system. A wet system that is not tuned correctly can result in dangerous happenings. Say in a nitrous backfire, where timing was off, fuel supply was not adequate, causing detonation. Take a look at this.
http://files.automotiveforums.com/gallery/watermark.php?file=/503/6784nitrous_backfire-med.jpg