Well, since you need help...
In Diamond and Pearl there are three categories that an attack is classified as, Physical, Special, and Other.
Defense is for Physical Defense, this means if someone uses a Physical attack on your Pokemon it will help your Pokemon resist receiving as much damage when you're attacked.
Special Defense is for Special Attacks, this means if a Pokemon attacks you with a Special Attack, it's damage will not effect it as much because it can resist it better...
Other categories for moves are defined by types, there are 17 types in Pokemon... there is an 18th, but it's not truly a type, it's just a move type "???"
All these types alone have certain weaknesses and resistances... I'd go more in depth but there is an application in the Poketch that lets you choose an attack type and choose the opponents type to see how much it would effective if at all..
if you still need help check out these pages... they were all written for the GBA/GBC games and haven't been updated for the 4th generation as far as I know.
http://serebii.net/games/battle.shtml
http://serebii.net/games/type.shtml
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