Psychic, Poison, Dark, & Ghost Types

Psychic Types

Psychic type Pokemon tend to be very powerful Special Attackers and Special Defenders, and are very much a force to be reckoned with.  There aren't a whole lot of Psychic types, and fair sum of them tend to be Legendary Pokemon (extremely rare Pokemon).  Having a Psychic type Pokemon in your team can add a significant edge.

Strong Against: Fighting, Poison, 

Bad Against: Psychic, Dark (no effect), Steel

Weak Against: Dark, Ghost, Bug

Resistant To: Psychic, Fighting

Psychic types have no effect against Dark types, so make sure to watch out!

Poison Types

Poison types specialize in slowly bringing an opponent down in HP (hit points).  They are only strong against one type, but they can be a very good addition to a team by being able to poison your opponent's Pokemon.  Poison types cannot be poisoned themselves, so there is no worry if your opponent happens to have a Poison type or use Poison type moves.

Strong Against: Grass

Bad Against: Poison, Ground, Rock, Ghost, Steel (no effect)

Weak Against: Ground, Psychic

Resistant To: Grass, Fighting, Poison, Bug

Poison type moves have no effect against Steel types, so make sure to watch out!

Dark Types

Dark types are the bane of Psychic types, as well as Ghost types.  Psychic type Pokemon have no effect on Dark types, and Ghost types will not do very much at all.  Every Dark type is different, and there have been a wide variety of new Dark type attacks, so having a Dark type Pokemon in you team can be very helpful, especially when facing a powerful Psychic type.  But like Psychic types, Dark types are weak against Bug type attacks, so be careful!

Strong Against: Psychic, Ghost

Bad Against: Fighting, Dark, Steel

Weak Against: Fighting, Bug

Resistant To: Ghost, Dark, Psychic (No Effect)

Dark types are also weak against the powerful Fighting type, so be wary when facing them!

Ghost Types

Ghost types are tricky.  They use moves that can hurt themselves to hurt their opponent, and also just be a nuisance.  Ghost types vary from one to the next, but they all tend to learn the same aggravating moves such as Curse, which cuts the Ghost type's HP in half to put a curse on the opponent, reducing their HP by 1/4 every turn.  A Ghost type Pokemon can put a unique curve on your team.  The only problem with Ghost types is that they do not effect Normal types, which there are many of in the game.

Strong Against: Psychic, Ghost

Bad Against: Dark, Steel, Normal (No Effect)

Weak Against: Ghost, Dark

Resistant To: Normal (No Effect), Fighting (No Effect), Poison, Bug

As I said before, Ghost types are annoying to fight, because both Normal and Fighting type moves cannot touch them, making them very fun to use in certain circumstances.