Dry kits will spray nitrous into the intake stream before the throttle body with no feul. dry kits usually however come with a nitrous regulator that shoots around 20+ psi of nitrous to your fuel pressure regulator causing an increase in feul pressure to make up for the added o2 in the nitrous. This is good for NA cars using less than 100 hp increase with nitrous

Wet kits are like dry kits however instead of increasing the fuel pressure they spray fuel into the intake with the nitrous.

Direct Port is when the nitrous is sprayed into your intake manafold runners each runner has a small nitrous nozzel that is tapped into the manafold right before the head causing all the cylinders to have an equal nitous charge.